A feature common to all caudiciform plants is the need for alternating growing and resting seasons. The cycle is annual in cultivation, although in habitat the dormant periods may last longer and are determined by the erratic intervals between rains. During the rest period the temporary photosynthesizing organs die away or are at least reduced, so no watering is needed except the slightest damping to keep the roots from drying back too drastically. One species differs from another in the timing of the dormant season, and rather than go by the calendar it is better to gauge watering by the appearance of new growth or the dieback of old. Some, such as Dioscorea, seem to go their own way and may even resume growth on an old stem before it has died back completely.
As regards hardiness, caudiciform plants run the complete range from tropical and tender to the hardiest of all succulents, the circumpolar Sedum rosea. A beginner with an average cactus collection anxious to join the cult would be recommended to sample the following, which should thrive if the winter minimum temperature is set at 5°C (40°F):
Kedrostis, Cyphostemma, Pelargonium. Nolina, Pachypodium, Dioscorea. Fou-quieria and dwarf Cotyledons.
Right (21.23): A section ot one glasshouse al Heidelberg University devoted to caudicitorms-perhaps the largest collection ol them in Europe
Below right (21.24): Bursera odorata. like Pachycormus. (21.1. II) is a grotesquely obese shrub in age. but small specimens, which are not easy to find, delight ihe lover ol bonsai plants.
Below (21.25): The caudex in Euphorbia silenitolia is wholly below ground, although in cultivation it is usually planted al a higher level as a safeguard against waterlogging
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