Dierama cultivation and propagation
Like so many plants from South Africa, dierama come from areas of summer rainfall. That means that they like moisture in the growing season, particularly in mid-summer when they flower, but are mostly used to dry conditions in winter. Dierama corms resent waterlogged conditions in winter so they will not grow well in heavy or clay soils. They will grow quite satisfactorily in light free draining soils, but which do not dry out in summer. Dierama do not seem to be particular as to the pH of the soil, they are tolerant of an alkaline soil though a neutral to acid soil is likely to suit them best, particularly as some species occur naturally in peaty areas in the wild. They do not need a rich soil but will not resent a little feeding, periodically. However, it is more important to periodically re-invigorate clumps of corms by digging them up and splitting them, removing the old dead basal corms. The corm should then be replanted between 50-100 mm deep, depending on the size of the corm, and 50 mm or more apart to allow for each corm to clump up, without over-crowding its neighbours.
Dierama are quite amenable to being grown in pots, though deep pots, such as long toms are preferable. When planted in the open ground they will, rather like bluebells, gradually bury themselves deeper into the ground. They do however employ a rather different method. Each year a large white fleshy tapering root develops downwards from the corm, called a contractile root. When this root dies back it shrivels to a narrow wiry root and in the process it pulls the corm deeper into the soil. New corms form on top of or to the side of the older ones, and when clumps become over-crowded the new corms gradually reduce in size and stop flowering.
The evergreen foliage is initially upright but can become lax with age. The older leaves die back but do not rot quickly and can accumulate, making the clump very congested. The old leaves are best periodically removed, for if left to accumulate they can eventually almost choke the centre of the plant, depriving new foliage of light, and will eventually cause the clump to collapse. The old leaves are remarkably tough and are best cut off close to ground level, rather than trying to pull them out, which can result in cut hands or damaging or even the removal of the growth point. In the wild some species are adapted to Veld fires which, along with the grass, burn off the old foliage and even the live foliage, which quickly re-grows at the outset of the wet season.
The collapse and apparent death of an old congested clump is not as disastrous as it may appear. After removal of the dead and dying foliage, the corms should be lifted, split and replanted as mentioned above and this will give the younger corms a fresh start.
It is the peculiar characteristic of dierama that, despite being evergreen, they die back when transplanted. Only some of the smaller species may avoid this temporary loss of foliage, as will young seedlings before they have reached flowering size. This may well account for many gardeners believing that they cannot grow dierama, or that they have killed them when transplanting or dividing them. All that is needed is patience, and preferably a marker in the ground where the corms have been planted. It is likely that many impatient or despairing gardeners dig up the corms just when they are starting to re-grow and thus set them back again.
There is conflicting advice as to when the best time to transplant dierama is. The truth of the matter is that dierama corms can be transplanted at any time of the year. However, the timing will affect the re-growth of the corms. Basically, the re-growth or re-generation of dierama corms is dependent on soil temperature. Whether moved in autumn or spring, the corms will not start into re-growth until the soil is warm enough, usually in late spring. Within limits, the warmer the soil, the more quickly will re-growth occur. Transplanting dierama just after they have flowered, when the soil is warm, enables the corms to die back and re-start into growth before autumn. Once the corms have started into growth and the leaves have appeared above ground, they will probably continue to grow, even when the soil temperature has fallen below the temperature critical to trigger re-growth, but at lower temperatures growth will cease, and this will vary with species. Thus from the gardeners point of view summer transplanting just after flowering is usually best.
One way of speeding up the re-growth process, is to transplant the corms into pots which can be brought inside to warmer temperatures; this will accelerate the re-growth. Care must be taken not to disturb the roots too much when planting out the forced corms in the garden, as this could result in setting off the re-growth cycle again. The time required for dierama to flower again after re-planting is related to the overall size of the plant. The small species will flower much more quickly that the larger ones, all other things being equal. Some small species can be transplanted in early spring, and given warm enough conditions will flower the same summer, usually in July, or August for most species. The larger species will take an additional year to flower. When corms are transplanted, they usually start to make offset corms when re-growing so that once flowering starts, there will usually be a number of flowering stems which will increase annually until the plant is in decline. For continuity of flowering of dierama, it would be best to prise off some corms from a maturing clump and replant them a year or two in advance of replanting the whole clump, which can then be undertaken once the earlier detached corms have started to flower again.
Dierama are easily raised from seed, which is usually produced in abundance in the attractive spherical seed capsules which burst open on maturity, displaying the orange-brown seeds for quite some time before they eventually drop out. Seeds can be planted directly in the open ground, and germination and some initial growth often takes place in early autumn before the onset of winter. Alternatively seeds can be sown indoors when ripe in late summer or in spring. The relatively large seeds should be reasonably spaced out in the pot, so as to make pricking out easier. As dierama seeds, especially from cultivated plants, have a high germination rate, several seeds can be planted, well spaced out, in larger pots so as to avoid the need for pricking out, and the slight set-back the seedlings experience. A pot of such seedlings can be planted out in late spring or early summer, where they will grow on and form a clump.
Seedlings can usually be transplanted fairly easily before they reach flowering size, without risk of them dying back. The seedlings should be planted in fairly deep pots as even small seedlings can develop contractile roots to pull the corms down deeper. Seedlings in deep pots will grow away better than those potted on in relatively shallow pots. Once the young plants have reached flowering size, corm disturbance will result in die back of the foliage. Inevitably to the chagrin of gardeners, self-sown seedlings always seem to grow better and faster than those sown inside, potted on and cared for. Dierama can and do self-seed in the most unlikely places, even in the centre of established plants such as agapanthus or kniphofia, and they will prevail to the detriment of their hosts. Gravel is an ideal host for self-sown seedlings as are cracks between paving slabs and it is surprising that quite large clumps can develop in very narrow cracks, and necessitate the lifting of the paving to remove them.
The mature corms of dierama will withstand being dried out for a short period, though juvenile corms will not.
Most species of dierama clump up fairly readily, though in general the larger the species or cultivar is, the slower it is to clump up. Hybrids with the small clumping species such as D. dracomontanum usually adopt their characteristic vigour. Dierama can be increased by division, though this is slow process, with die-back and re-growth as described above.
First published in the Half-Hardy Group Newsletter, Spring/Summer 2008
and subsequently in Cornucopia Issue 27.
© Copyright for this article: Gary Dunlop
This article was taken from a copy of Cornucopia that was published in 2011. You could be reading these articles as they are published to a national audience, by subscribing to Cornucopia.