The genus Trillium contains members native to both North America and Asia but this article focuses very much on those species found in eastern North America. It could just as easily have been entitled ‘Some of my Favorite Trilliums', and is intended as an introduction to some of the Trillium treasures rather than a comprehensive survey of the genus. Two books have recently been published which do exactly the latter, details of which can be found at the end of the article.
My first experience with trilliums came in the UK when I tried to grow some of the more common species in pots, but since moving to Pennsylvania in the north-eastern USA in 1995, I have concentrated on growing them in the open garden, and studying them in the wild. As with any other group of plants, they have their likes and dislikes, and there are certain tricks to be learnt if one wishes to cultivate and propagate them successfully. In the following pages I hope to share these experiences with you and pass on some of the excitement that goes along with growing trilliums.
You may be wondering exactly what an article about trilliums is doing in ‘Bulbs'! Although not bulbs in the true botanical sense, trilliums are undoubtedly geophytes in as much as they possess a storage structure (the rhizome) to see them through seasons which are unsuitable for, or hostile to, growth above ground. Depending upon species this can be as long as from June to the following March. Rhizome size, shape and degree of branching can vary dramatically from species to species, and can be a useful aid in identification and a source of propagules, if treated correctly.
Trillium is far from unique in that the taxonomic status and nomenclature of many species is somewhat controversial and in a state of flux. Depending upon your preferences for lumping or splitting, a good number of species and sub-species could be reduced to synonymy. Often known by the common names of ‘Wake Robin' or ‘Toadshade', all species have horizontal subterranean or surface rhizomes, three leaves, three sepals, six stamens, three stigmas and the American ones all have three petals. The fruit is always a berry-like multi-seeded capsule, which falls from the stem when ripe. Presently between forty and forty-five species can be recognized. However, it is quite clear that all trilliums can be conveniently described as either sessile or pedicellate, i.e. the flower either sits directly on top of the leaves (sessile, subgenus Phyllantherum ) or has a pedicel (pedicellate, subgenus Trillium ). In the latter, the flower may be erect or held below the leaves. This makes for a reasonable starting point in attempting to sub-divide the genus into more manageable groups. The sessile and pedicellate trilliums are dissimilar in a number of ways pertaining to the leaf form and coloring, and especially the flower structure. Both groups are indispensable elements of the woodland flora, providing superbly attractive foliage and flowers over several months in the spring, the sessiles generally flowering earlier than the pedicellates, often starting in late February or early March, depending upon location.
Trilliums make wonderful associations with other native and exotic woodland bulbs and herbaceous perennials. In the garden they mix particularly well with Corydalis, Dicentra, Disporum, Erythronium, Hepatica, Iris, Jeffersonia, Mertensia and Sanguinaria. Three spectacular wild locations I especially remember were one in north-central Alabama in which Trillium flexipes grew with Phlox divaricata, huge patches of Jeffersonia diphylla and white Dodecatheon media; a second in north-western Illinois where tens of thousands of the Snow Trillium ( Trillium nivale ) grew with Dicentra cuccularia, Dodecatheon amethystina and the most wonderful clumps of Hepatica acutiloba in fabulous color forms; and a third in northern Georgia in which Trillium decumbens cohabited a steep hillside and ravine with yet more superb color forms of Hepatica acutiloba , Disporum maculatum , Phlox divaricata, Disporum lanuginosum and Iris cristata. Erythroniums are rarely absent in trillium country and all three of the above locations had their quota.
In the wild most species are found associated with mature deciduous forests, generally in vernally moist habitats which dry out somewhat during the summer months. Trilliums are frequently found on the cooler slopes and terraces leading down to creeks and rivers, some species are also very happy in areas which are seasonally flooded. Conditions in the wild, most importantly, favor seedling establishment as well as meeting the needs of mature plants. In contrast, in cultivation, adult plants of most species are highly adaptable and will often thrive for long periods in seemingly unsuitable conditions. Like many woodland plants, as long as there is plenty of moisture when in active growth, trilliums will take quite a lot of sun, although under such conditions seedlings are unlikely to establish. Generally speaking, seedling establishment needs correct soil temperatures, pH, drainage and moisture and freedom from plant competition. Specific habitats will be discussed in the context of descriptions of the species, general cultivation requirements will be discussed in the section on cultivation and propagation, as will the relatively few pests and diseases which affect trilliums.
Selected Species
Most, but by no means all, of the species are briefly described below. One of the main aims of this article is to increase awareness of the beauty and garden value of trilliums, especially some of the lesser known species, so the descriptions will focus on garden worthiness rather than botanical considerations. Where possible, photographs of most of the species have been included. Although a number of cultivars have been described (many with the ‘Eco' prefix from Eco Gardens, Atlanta, Georgia), none of those will be discussed here. The main reason for this is that, at present, there is no reliable method available for the clonal production of trilliums on a large scale. Thus, even though cultivars have been named, they are effectively unavailable. Where plants have been grouped together, below, this is for reasons of convenience rather than any taxonomic affinity.
Sessile species - subgenus Phyllantherum
Trillium decipiens, Trillium reliquum and Trillium underwoodii
These three species have a great deal in common, inhabiting typical trillium habitat in the deep south of the USA, particularly parts of southern Alabama and Georgia. They are especially worth growing for their stunningly beautiful leaves blotched in every shade of green overlaid with streaks of cream and silver. This description does them a woeful injustice, only a photograph can adequately demonstrate their beauty. Of particular note is the fact that the leaf markings remain fresh and bright from unfurling to dormancy. This is in sharp contrast to species such as T. cuneatum , whose beautifully marked fresh leaves rapidly fade to muted greens and browns. Their flowers are also very similar, being erect and colored from a dark brown-maroon to a rich red-maroon. This description, again, does them little justice so I'll defer to the photographs. As is the case with flowers of all sessile species, the colors fade with time but they are exceptionally long-lived, lasting until the seed is ripe.
It is virtually impossible to distinguish between non-flowering plants of T. decipiensand T. underwoodii. The major difference is one of stature of flowering plants, and the ratio of leaf length to stem height. In T. underwoodii the stems are 3” to 8” tall and 1 to 1.5 times the leaf length, whereas in T. decipiens the ratio increases to over 3, the leaf size remaining similar. The latter is a truly elegant plant. Until a spring 2003 trip to Alabama I wasn't convinced I had ever seen true decipiens , but I certainly have now! After scrambling up a mini cliff into the woods above the bank of a suitable looking creek I was thrilled to see odd scattered plants but was literally stunned to see the sight that unfolded in front of me. Spread over the slope, in an area perhaps 30 yards by 400 yards, was an unbroken sea of at least 20,000 plants of classical T. decipiens . The majority were flowering to perfection. They were 12-18" tall, with the leaf span in perfect proportion. The flowers ranged in color from 'normal' dark maroon through paler bronzy-maroons to olives, bicolors, muddy yellows to a beautiful clear lime green. In the latter case they were also devoid of all dark pigmentation in the stems. Sepal colors also varied a lot, some being very dark. It was one of the most incredible sights I have ever seen.
T. reliquum is an almost decumbent plant, the leaves sitting on or just above the forest floor. It is superficially like T. decumbens at first sight but differs in a number of features, particularly its non-pubescent stem. An S-curve in the stem is often quoted as being diagnostic but in my experience this character is not reliable.
Trillium decumbens
Probably my favorite sessile trillium, T. decumbens lies flat on the forest floor and possesses a character all of its own. It is found in northern Alabama and north-eastern Georgia. A memorable day in spring 2002 found us looking for it in the area of the Black Warrior River in Alabama. We had parked on the nearest roadside and headed off towards likely habitat, only to find the river was a lot further away than we thought. Finding a few plants in a small side valley gave us hope but it was getting dark and the severe weather that had been promised was on the way. Suffice to say that we scrambled across creeks, through woods and brush and across fields and ended up nowhere we should have been and not sure where we were (and the GPS was in the car). It was getting very dark and raining heavily when we stumbled into another steep ravine to find it full of tens of thousands of plants of T. decumbens - 'locally abundant' as the books would have it. It was quite bizarre to see these plants, in full bud, and some the size of an outstretched mans hand, eerily lit by the bright pink lightning that was being generated by the mother of all storms that was now nearly overhead. We headed out and eventually found the car in pitch blackness and soaked to the skin but vowed to try to find them again the next morning. Without doubt they had never been found previously, unseen except maybe through a hunter's scope. The creeks were seriously flooded following the storm but we did find the plants again and were amazed by them all. Most were in tight bud but some just starting to open. A very few plants had almost pure silver leaves and the markings on others were fabulous. The open flowers are a glossy red-maroon and the petals are attractively twisted at the tips.
Trillium discolor
Another favorite, this time hailing from the upper drainage of the Savannah River on the border between S Carolina and Georgia. In comparison with the other yellow sessile trillium, T. luteum, this is a truly refined species and quite distinct. Although generally dwarfer in all proportions than T. luteum , very robust individuals can exist in the rich flood plains it inhabits. It can also be incredibly abundant; I have seen it on the banks of small creeks in areas so dense it is impossible to walk through them without damaging the plants. Unlike the acid yellow of T. luteum, the petals of T. discolor are a soft creamy yellow and upon seeing large colonies of them, the effect is one of seeing thousands of candles. The petal shape is unique in that they are quite spatulate, much broader at the tip than the base, and the stamens are purple. They can be delightfully but not strongly fragrant of lemons and particularly nice forms may have strong red flares extending up from the base of the petals.
Trillium foetidissimum
Found in Louisiana, in rich ancient woods east of the Mississippi River, this species is relatively invariant in flower and leaf, although as is the case with many red-maroon flowered sessile species, pure yellow forms can very occasionally be found. Similarly, forms also can be rarely found with all-over silver leaves. In size and proportion it is similar to T. discolor and quite charming. With dark red-maroon flowers, it has strongly mottled leaves, in flavors of green and bronze, but without the silver patterns of T. underwoodii. It is true to name, the flowers emitting a fetid odor which is especially noticeable on warm still days. Of all the trilliums discussed, this species suffered the most in the garden here during the very hard N American winter of 2002-2003. It was the first to emerge in mid-March and a number of shoots were badly damaged by freezing in more exposed raised beds. However, plants in more sheltered spots were fine.
Trillium maculatum
Another special sessile species, this plant grows over a wide range across central Alabama, Georgia and S. Carolina. The ‘maculatum' refers to the leaves, which can be strongly and darkly mottled. The flowers are usually relatively tall and a very clear red-maroon, lacking the brown overtones that flowers of many sessile species can have. Yellow forms exist, and if you are incredibly lucky you might find the incredible T. maculatum f. simulans with yellow and maroon bicolored petals. Plants can be very robust and possibly confused with T. cuneatum, although the ranges of the two plants do not overlap. Many sessile trillium species can be mis-identified if one is not familiar with them, particularly when isolated in cultivation. Even trillium experts are often heard asking ‘where in the wild did you find it' when asked for an identification of a garden plant! Having the benefit of seeing many species in the wild certainly allows one to get a feel for the true differences between the species.
Trillium lancifolium, Trillium recurvatum and Trillium stamineum
Although these three species are related they are very easily distinguished from each other and from other species. None can be described as showy but each makes a unique contribution to the genus. They are also interesting because they have a natural propensity to branch and form clumps, something that most trilliums will not normally do.
T. stamineum is a medium sized species with relatively small flowers, the fragrance of which can be rather unpleasant. However, the thin dark maroon petals are horizontally inclined and uniquely twisted along their length. Less clump forming than its two relatives, it is found in central and western Alabama, eastern Mississippi and west-central Tennessee.
T. lancifolium also has twisted petals in many forms, but these are very long, thin and erect. It also has a unique look, taking its name from the lance-shaped leaves. The stems can be very tall, easily up to 18”, and 2.5 to 3 times longer than the leaves. Flower color appears very variable, from dark maroon, through bicolors to almost green. The rhizome is also very unusual, being long and thin, and very brittle. It also tends to branch and form tight clumps more frequently than most other trilliums. In habitat it is more often found closer to small creeks, usually on the flood plain and in areas that can be totally inundated for several days or weeks in particularly wet springs. However, it doesn't have an obligate need for such damp conditions, behaving perfectly well in cultivation in much less wet spots. T. lancifolium is unusual in its disjunct distribution. In addition to the ‘normal' populations in central Georgia and Alabama, there are populations in the Florida panhandle near the Chattahoochee River which flower very early, before the end of February in many years.
T. recurvatum is also easily identified with its petiolate leaves, recurved sepals and relatively short petals. It has a similar color range to T. lancifolium , although is normally a brownish-maroon to red-maroon. It has a very wide range for a sessile trillium and can be quite variable in size, petal color and leaf marking. T. recurvatum f. shayi is a yellow flowered form which occurs with a relatively high frequency in certain populations and when compare with equivalent forms of other sessile species with yellow variants.
Trillium cuneatum, Trillium ludovicianum and Trillium gracile
T. cuneatum grows over a vast area and is easily the largest of any eastern sessile trillium. Growing mainly in upland slopes and woods, it could be described as coarse when compared with the smaller species but it is particularly spectacular when seen en masse. Very many different forms occur over this range and these can be confused in cultivation with other species. Petal color varies from yellow through green to brown and deep maroon; leaves can be pure silver, green or strongly mottled.
T. ludovicianum and T. gracile definitely fall into the category of hard to distinguish between if you don't know their origin! Originally thought to be confined to the state of Louisiana, it appears that T. ludovicianum may well grow as far north as Chattanooga, Tennessee, where it co-exists with T. cuneatum. It is a more refined plant with a clearly distinct narrower petal shape but may well intergrade with T. cuneatum over some of its range. The range of petal colors is similar to that of T. cuneatum and the leaves are very attractively mottled. T. gracile is geographically distinct, with a narrow distribution in western Louisiana and eastern Texas, but is morphologically not particularly easy to distinguish from its nearest neighbors, T. ludovicianum and T. foetidissimum . There seems to be less variation in flower color, generally a brownish-maroon, but, highly characteristically, it does flower several weeks later than these species, often coming into flower when they are already well past their best. I saw T. ludovicianum and T. foetidissimum in full flower in the wild in the first week of March in 2002, but T. gracile was still in immature and very tight bud at the same time. It is the last sessile species to flower here in SE Pennsylvania.
Other sessile species
Of the eastern sessile species, only T. luteum, T sessile, T. viride and T. viridescens have not been specifically discussed above. T. luteum is a large relatively invariant, yellow, flowered species, whose distribution is centered in eastern Tennessee, particularly in and around the Smoky Mountains. It is probably the most common trillium in cultivation and widely recognized. T. viride and T. viridescens are both green flowered, sometimes with a purple base to the petals in the latter case. The latter two are frequently confused in cultivation and are not particularly showy, although they are large plants. T. sessile is also frequently misrepresented in cultivation; most of the plants bought or seen under actually correspond to T. cuneatum or one of the western sessiles, although the true plant is of much smaller stature than any of these. It can be a charming plant and very fine forms with excellent leaf mottling and petal color can be found. In particular, I have seen yellow plants, and those with yellow flowers edged with purple, creating a very attractive picotee effect.
With profuse apologies to the western US sessile flowered species, I won't discuss them in this article. There are a number of very attractive species (some with white flowers), generally very large of stature, but the few plants of T. chloropetalum I have tried to grow here have fared relatively badly and I have little or no practical experience of the others, which include T. albidum, T. angustipetalum, T. kurabayashii, T. parviflorum and T. petiolatum . They are, however, at least as confused, taxonomically and in cultivation, as the eastern sessiles!
Pedicellates - subgenus Trillium
Trillium catesbaei
T. catesbaei is an exceptionally classy plant, so what better one to start with? Growing widely in northern Georgia and north western parts of the Carolinas it is often found with T. luteum and T. rugelii or with T. discolor. It is a relative loner in the wild, individuals are often widely scattered, and does not form clumps in cultivation. The leaves appear distinctly petiolate and are plain green and unmarked, as is the case for all of the eastern pedicellate species. The flower can be held above (rarely) or below the leaves and is quite variable in size. The petals can be white or pink, and any shade in between, and some forms have superb deep rose pink petals which are strongly reflexed and perfectly compliment the prominent yellow stamens. The pedicellate species generally flower a month or so later than the sessiles and T. catesbaei is at its best from early April to June, depending upon location.
Trillium cernuum and Trillium rugelii
These two species are very similar and have been considered as northern and southern ‘forms', respectively, of a single species. T. cernuum has white, strongly recurved petals in a flower held tightly below the leaves and is the most northern of American trilliums, growing from northern Maryland northwards. Its habit of hiding its flowers beneath a large leafy canopy makes it not particularly showy and the plant also performs poorly in cultivation in lowland areas south of its natural distribution. It will often form small clumps of 3-5 stems, as will T. rugelii, which has a distribution centered on the southern Appalachian mountains of Virginia and the Carolinas. The white stamens of T. cernuum are purple in T. rugelii and the ovary is also purple or white with a purple stigma. The flowers of T. rugelii can be much larger, as can the plants generally, but they remain relatively non-showy because the flowers are similarly hidden by the leaves.
The sessile flowered trilliums described above do not appear to hybridize widely in the wild (or in cultivation) but the same can hardly be said for the pedicellate species, especially those related to Trillium erectum. T . rugelii falls into the latter category and hybrids are well known in the wild and cultivation, and can be very attractive plants. In Brevard, N Carolina, in a filthy ditch behind a strip mall, I have found both T. rugelii and T. vaseyi and every possible variant in between. The mixture of the large deep red-purple flower of the latter, with the white flower of T. rugelii makes for some lovely combinations, including true bicolors and white flowers with red veins.
Trillium pusillum
T. pusillum is a beautiful species which tends to flower rather early for a pedicellate trillium. Unfortunately it is a rather confused little fellow, or at least it has managed to confuse a large number of botanists. It occurs in several widely disjunct populations west from eastern Maryland to the Ozarks of Missouri, south to western Texas and then back east through Alabama and Georgia into S Carolina. There are currently 6 or 7 published varieties, but the distinctions between many are blurred and this is not the place to go into a discussion of them. Suffice to say it is always very charming, dwarf (3” – 12”) and has an upwards facing white flower. The flower often has ruffled margins and, in the manner of Trillium grandiflorum, fades to rose pink as it ages. The foliage of some forms is almost purple in color until it matures to a deeper green. In the wild it is frequently found in seasonally boggy or even swampy areas, but doesn't require such conditions in cultivation.
Trillium grandiflorum
If T. luteum is the best known and mostly commonly encountered sessile trillium in cultivation then T. grandiflorum is certainly its pedicellate bedfellow. It surely needs little description and is widespread in the wild north from Tennessee to the Canadian border. Although very common, it is a superb garden species, robustly growing to 15” or so with pure white flowers which fade to rose pink as they age. As might be expected for such a widespread and much grown species, a number of forms have been selected. These include several distinct double white forms which are very beautiful, ‘Snowbunting' is perhaps the best known, although it has to be searched for, and commands very high prices! Also much in demand are the pink forms (forma roseum ) which occur infrequently in the wild, but especially in the mountains of Virginia. There has been much discussion about what constitutes a true pink form, and it can be confusing as all T. grandiflorum eventually age to deep pink. True f. roseum opens pink or, if it opens pale pink, shades to deeper pink within a matter of a day or so. Also available is a dwarf form of T. grandiflorum , perhaps only half the height of the normal plant but with quite large flowers. This appears genetically stable and common in certain locations, but it has not been formally recognized.
Trillium nivale
The Snow Trillium is by far the earliest flowering trillium here and a wonderful showy plant. Absolutely hardy, as might be expected as its heartland in the wild centers on Indiana, Illinois and Iowa, it frequently gets covered in snow whilst flowering and seems immune to being frozen solid. Here in SE Pennsylvania it flowers in early to mid-March, long before most trilliums are even through the ground. The flowers are pure white to creamy-white, of heavy texture and prominently veined, and replete with beautiful golden yellow anthers. Although it is typical of the pedicellate trilliums in that its leaves are unmarked, they are unique in that the best forms have leaves which are a wonderful shade of blue-green, overlaid with a pewter caste, often with the veins picked out with silver speckles. All of this at a height from 2-6”, at most, making t. nivale one of the most desirable trilliums for the garden.
Trillium erectum, Trillium flexipes, Trillium simile, Trillium sulcatumand Trillium vaseyi
This is the section that could get me into real trouble! I'm probably on safe ground starting with the descriptions of the ‘true' plants, but you'll soon get the message that you may have more chance of winning the lottery than correctly identifying your pedicellate trillium from this group. All of the species have large plain green leaves. T. erectum can be anything from 8 – 24” tall, with flowers, flat and wide-spreading in profile, of white, red, maroon, yellow-green or red-brown, the petals frequently tending to be lanceolate in shape. The flowers can be erect, straight out sideways or declining. T. flexipes is usually 15 – 18” tall, traditionally white flowered with broader petals on erect flowers, although red forms have been described, as have forms with declining flowers. I haven't mentioned ovary color yet – but this can vary from white through cream to pink-purple. T. simile is a very attractive large species, often forming small clumps, with erect creamy-white flowers (normally) with a purple ovary. The flower is characteristically more funnel-shaped with the ends of the petals flaring sideways. T. sulcatum is of similar stature and equally showy but has dark red-maroon to purple flowers with broad petals that are slightly flatter than those of T. simile. Of course occasional white, pink cream and yellow forms can be found together with beautiful bicolors and picotees. Last but not least is T. vaseyi, a spectacular species with huge (up to 3” across) flowers of deep red-maroon nodding below the leaves. White flowered forms are known and it is the last species to flower here.
All of these subjects make superb garden specimens and it is important not to get too hung up on nomenclature. The difficulty in identifying these species in the wild, because they are so naturally and extremely variable, is compounded by the fact that hybridize in cultivation, and in the wild where their distributions overlap. Photographs of a number of these species and forms are included to exemplify their variability and beauty.
Other pedicellate species
Of the eastern pedicellate species, only T. persistens and T. undulatum have not been described. The former is federally endangered and also the least showy of all the species in this group, whereas the latter is the probably the most difficult trillium to grow outside of its native range. The western USA and Asia has its share of pedicellate species, but, as is the case with the sessiles from the west, they do not generally perform well in cultivation in the eastern USA, for reasons which are briefly discussed below. Other species include T . ovatum, T. hibbersonii and T. rivale (included by some in the new genus Pseudotrillium ) from the USA, and T. apetalon, T. camschatcense, T. hagae, T. smallii and T. tschonoskii from Asia.
Cultivation and Propagation
Cultivation
As alluded to above, the cultivation of many trilliums is generally not problematic and they are actually very adaptable plants that are quite difficult to kill! They do, however, resent disturbance and will take a year or more to settle down after being moved. Most are well served by moisture retentive but very well drained compost (in pots or in the garden) which is vernally moist but allowed to dry significantly in the summer when the annual growth cycle ends and the plants go dormant. There are numerous ways to accomplish such a soil mix, naturally or artificially, any are likely to work that effectively mimic the conditions in the wild. Although they are essentially woodland plants, many trilliums exposed to quite strong sun in early spring whilst they are leafing out and the flower buds are maturing, and before the tree canopy has expanded. The species from the deep south, especially, will encounter temperatures well above 70F during this period, but the shade of the trees brings welcome relief when the temperatures become searing later on in the summer. The majority of species will have completed growth, set seed and gone dormant by early-mid August in SE Pennsylvania, although this is very much dependent upon the local climate. In the deep south they are gone much earlier. Those immature plants which don't flower, and mature ones which don't set seed, tend to go dormant as many as 4 – 6 weeks before those that have set seeds. Presumably the maturing seeds capsules produce inhibitors to prevent onset of dormancy until they are ripe and the seeds dispersed. Although not proven, I suspect that the deep south sessile species are well adapted to hot and relatively dry summers and they benefit from these conditions in cultivation. Thus they are possibly not the best candidates for more regularly moist shade beds in areas such as the UK, which have generally cooler and moister summers. On the other hand, the pedicellate species from the Appalachian Mountains and northwards will probably flourish better in such conditions which are closer to those they encounter in the mountains and coves they inhabit, and certainly seedling establishment will be favored. Trilliums from the western USA and Asia will certainly grow better in such maritime climates than they will in the eastern half of the USA.
Some trilliums, particularly T. nivale, grow in limestone areas in the wild, but neither an alkaline pH nor the addition of limestone are generally necessary in cultivation as long as the plants other requirements are met.
As mentioned above, T. undulatum is almost impossible to please when conditions do not mimic those of its home range. It is essentially a northern plant, deigning only to come south at the highest elevations, and therein lies the solution to keeping it happy. It appears to be one of those species that is a cause of extreme frustration to growers who inhabit warmer lowland locations, albeit ones which can get very cold in winter. Absolute temperatures are not the key, rather the ability to provide a significant diurnal temperature variation. Here the summer time temperatures average the high 80s and 90s F during the day, and maybe 10F lower at night, whereas places in which T. undulatum does well will have similar, if slightly lower, highs, but the nighttime temperatures will drop to the high 50s or low 60s F. The hot muggy nights they despise effectively ‘choke' the plants by inhibiting the physiological processes which occur at night. Whilst T. undulatum, is an extreme example, a number of the more northern species such as T. cernuum and T. nivale , will not generally fare well in lowland areas in the southern half of the USA, for much the same reasons.
Propagation
Trilliums, unfortunately, have so far not made good candidates for rapid vegetative propagation, either by division or tissue culture, and this effectively limits larger scale production. However, they do set copious seed, and this affords the best means of increase. The down-side is that most species will take 1 – 3 years to germinate and a further 5 – 7 years to flower! Hand-pollination certainly helps maintain fidelity and increase seed set but most species will set seed naturally in the garden. I have never seen a specific pollinating insect but the sessiles in particular seem attractive to small flies. The ‘berries' ripen naturally by mid-summer in most species, although T. nivale is much earlier. When they are ripe the pods start to go very soft and they naturally fall from the stem and disappear rapidly once on the ground. Just before they are ripe they are frequently attacked by wasps attempting to get at the seeds, which have a fatty elaiosome to which they are attracted. In order to ensure I get to the seeds first and to try to keep the collection process as clean and effective as possible I collect the capsules up to a couple of weeks before they are fully ripe. At this stage the pods are still fairly firm and the seeds are a light brown color. I store the pods in plastic bags whilst they finish their disintegration process, and then rub the whole gooey mess through a flour sieve under running water, leaving only the seeds behind. At this stage various seed treatments have been recommended to reduce rot and germination time or increase germination percentage. Undoubtedly some work but all are time-consuming and I generally prefer to let nature take her course. Germination of seeds is best if they don't dry out, so sow the seeds immediately onto any suitable seed compost which meets the same criteria as described for mature plants, cover them with grit, and place the pots outside in a shady area and keep them moist. Alternatively, if you have the right conditions, you can sow the seeds directly in the garden. In my experience, only T. rivale germinates well from dried seed.
If all has gone well you can expect some germination the second spring after sowing, although this is frequently a relatively low percentage of the seed that was sown. Leave the seedlings in their pots to allow for further germination in subsequent years, and to allow the tiny seedlings to develop. They can be successfully moved into the garden or further potted on after 3 years or so, depending upon the species involved. I suspect you are starting to understand why seed raised trilliums are very expensive to buy – there's an awful lot that can and does go wrong in the time it takes from sowing the seed to selling the plant! Whilst seedlings are growing on they will benefit from regular weak feeding with a suitable fertilizer.
Some trilliums will form clumps naturally, as mentioned above. In the case of those that prefer to remain solitary, it is always worth seeking out individual clones that are genetically predisposed to clumping. Individual plants which are particularly attractive can be vegetatively propagated by disbudding the rhizome by either gouging out the terminal bud or cutting the rhizome into two an inch or so behind the terminal growth bud. I prefer the latter since the newly separated terminal bud with accompanying portion of rhizome will go on to make new roots and a shoot, and frequently also flower. In both cases, the loss of apical dominance causes dormant buds to sprout along the length of the remaining ‘headless' rhizome, and with time, these will develop into small plants which can be separated after a couple of years, if so desired. The timing of this exercise is important and it is best done shortly after flowering and certainly before dormancy – trilliums initiate growth of the roots that will support next years growth at this time and later disturbance will certainly set them back.
Pests and Diseases
Trilliums, like all plants, suffer from their fair share of pests and diseases. However, well grown plants are extremely tough and serious issues are relatively few. Seedlings of trilliums are no different to seedlings of other plants and similar care should be taken to ensure they don't damp off. Mature plants are generally trouble free but Botrytis fungal infections of the leaves and flowers can be a problem in very damp springs. Infection usually causes disfigurement of the leaves through the appearance of brown spots which can spread and coalesce, in severe cases causing defoliation and premature dormancy. The rhizome is not affected but obviously early dormancy robs the rhizome of the ability to build up reserves to support the following year's growth and flowering. Chemical treatments are available but not really practical for plants grown in the garden, I prefer to suffer the occasional damage and find no long term harm.
Cultivation in poorly drained composts and soils will certainly lead to bacterial and fungal rots of the rhizomes, which can cause loss of the entire plant. Interestingly, the rhizome of some species, such as T. sulcatum, can rot naturally in a controlled manner limited to the end distal to the terminal bud. Presumably this happens because that end of the rhizome can be many years old and is allowed to degrade naturally in a self-limiting manner which doesn't affect the rest of the rhizome, as long as other conditions are correct.
Mycoplasma (bacterial) infections can be serious, causing distortion and deformity of flower parts, and green streaking of petals. Without doubt this condition exists in the wild, most often seen in T. grandiflorum and T. erectum , but it is relatively rare in the garden. It would be wise to not grow (and destroy) any plants which show these symptoms, as there are no treatments available and mycoplasmas have the potential to be transmitted, especially by sap sucking insects. However, the discovery of the odd small green streak in the flower of an otherwise healthy plant is not justification for digging up the whole lot and burning them!
In certain damp climates slugs can be a problem and they will browse the emerging growing points in the spring, potentially causing serious damage. The usual preventative measures should be taken.
Bibliography
Trilliums by F.W. and R.B. Case (1997). Timber Press, Portland, Oregon, USA.
Trilliums in Woodland and Garden - American Treasures. D.L & R.L. Jacobs. Eco-Gardens, Decatur, Georgia, USA.