Cultivation in pots in the south-east of England
Contained within the flora of New Zealand are several genera whose members are true aristocrats of the select group of high alpine cushion-forming plants. Those of which we have experience, and will discuss in this article, include Raoulia , Haastia , Myosotis , Celmisia and Pygmaea. As the authors are two unashamed 'pot-freaks', we cannot offer advice on the cultivation of these plants in the open garden, or troughs or raised beds, but aim to pass on some of the tips which have allowed us grow these plants with some success in pots.
The general treatment given to all of the plants mentioned below is very similar. All are grown in a compost comprising around 60% sharp grit and 40% ericaceous compost; this seems to be more satisfactory than our previous compost which contained a proportion of the ericaceous fraction as John Innes 2 or 3. The single most important observation is that plants not potted on every year will rapidly decline in health & vigour. Feeding will not substitute for repotting, it appears that the roots need to spread into fresh compost. Consequently, regular repotting rather than initial over-potting is beneficial. Neither of us has bare-rooted Raoulia rubra yet to see if this is an alternative to potting-on !
None of the plants, with the exceptions of Haastia pulvinaris and Myosotis pulvinaris , are grown in the alpine house, other than as rooted cuttings. However, as there is some doubt about the cold hardiness (below -12 o C) of some Raoulias some plants are overwintered in an unheated greenhouse. Raoulia bryoides, eximea, grandiflora, mammilaris, parkii, petriensis, rubra & the hybrids x loganii and 'Margaret Pringle', together with Celmisia argentea and C. sessiliflora, Myosotis uniflorus and Pygmaea pulvinaris are all grown in clay pots plunged in sand in Access frames open to full sun & rain from late March until late October. If necessary during the summer, all the plants are watered overhead using a hose-pipe and rose. During the rest of the year they are given glass protection overhead but the frame sides are generally left open except in the severest cold or rain. With the exception of Celmisia sessiliflora they are kept moist right through the winter, the Celmisia is reputed to be more sensitive to moisture and is kept much drier.
Neither Haastia pulvinaris nor Myosotis pulvinaris have been grown outside although enough seedlings are available this year to attempt a trial. The latter plant seems very prone to botrytis during the dormant season & we doubt whether it will be happy outside at any time.
When grown in the ways described these plants are remarkably free from pests & diseases; botrytis is rarely a problem except on the Myosotis discussed above and aphids only seem to be interested in Myosotis uniflorus although a careful watch is needed at all times. Celmisia argentea only appears happy in a pot until it is about 6 inches across after which time it tends to woodiness and die-back & is best restarted from cuttings. Die-back is the most serious potential problem in all of these plants, in which case rosettes or clusters of rosettes, die for no apparent reason. Immediately cutting out the affected parts does not guarantee containment but makes the grower feel better ! In some cases the plant will completely fade away, in others the die-back will not spread & the damage will be filled in during the next growing season. A hole the size of a 50 pence coin was repaired in a 4 inch plant of Raoulia bryoides in one spring but the plant died (back) completely the following winter. It would appear that plants rescued from the hot & dry alpine house during the summer months do not suffer as badly as those incarcerated all year round.
It should not come as a surprise to hear that, as with most choice alpine plants, the key to success is regular propagation. With the exception of Raoulia grandiflora , none of the Raoulias nor the Haastia will flower in cultivation. Consequently, the only way to propagate cultivated plants is by cuttings taken at any time from the end of March until the autumn. Cuttings are taken as single rosettes, cleaned of any dead foliage, and put into fine sand watered from below then kept in a covered propagator. Time to rooting varies with the plant and the time of year - Raoulia x loganii taken in the late spring can easily root within 2-3 weeks but will root slowly over the winter if taken in the autumn. Approximately 40 cuttings of Raoulia rubra were taken at the beginning of April 1994 & around 30 had been potted up by the end of the summer, the earliest rooting after about 6 weeks. The last 5 rooted cuttings were potted up in March 1995 and are still in the propagator at the time of writing ! Rooted cuttings of this plant seem extremely sensitive to heat and had to be kept in the propagator for up to 2 months after potting up otherwise they would immediately close up & sulk. Cuttings of Raoulia grandiflora , taken with or without roots, seem to need similar treatment. The celmisias are also propagated from cuttings by necessity as they do not set viable seed. Pygmaea sp. can be propagated by cuttings or, most commonly, by division.
Obviously all of these plants can be propagated from wild-collected seed but it is not easy to obtain. We have raised plants of R. eximea and mammilaris and Haastia pulvinaris from wild seed but the numbers of plants resulting is few. All seed is treated in the same way & sown on the surface of a very gritty compost as soon as possible after the turn of the year. The seed pots are covered with a 2mm layer of grit then left outside, fully exposed to the elements, until germination is apparent at which time they are brought under cover. Germination is generally in the late spring following sowing but we have had an additional odd germination the following year as well so it is worth keeping pots for two years. The cotyledons tend to be very small & quite characteristic & confirmation of the identity of the seedling comes relatively quickly when a little piece of fluff appears in the crown ! We tend to prick out early, as soon as the seedlings are large enough to handle, and seedlings are then kept well watered and shaded for a couple of weeks. Raoulia eximea and mammilaris will make quite good sized plants of ten or more rosettes in the first year, Haastia pulvinaris tend to be more conservative. Myosotis pulvinaris and uniflorus are the most accommodating of the plants which we grow - they will root easily from cuttings and also flower and set viable seed in cultivation. However, home saved 1993 seed of the former, sown from the plant never germinated whereas 1993 SHS seed from the wild germinated immediately upon sowing after travelling halfway around the world !
The main difficulty in growing some of the choicer New Zealand cushion plants is in initially obtaining seed or plants. However, with care, observation of their needs, and regular propagation they can easily be grown to good-sized healthy plants, even in the relatively unsavoury conditions of the south east of England.