In horticulture, a volunteer is a plant that sprouts and grows without any action by the gardener. The implication is that volunteers are desirable plants, which distinguishes them from undesirable weeds. There’s sometimes a fine line between the two. In my outdoor garden, Vernonia glauca (broadlead ironweed), Poncirus trifoliata (trifoliate orange), and Callicarpa americana are a little too enthusiastic about seeding around. When another gardener wants to trade for the seedlings, they’re volunteers. When I have to dig them out of the wrong flower bed, they’re weeds.
In my greenhouse, several species have established themselves as volunteers. They can pop up almost anywhere, but they never choke out the rightful inhabitant of the pot and are very interesting in their own right.
Dorstenia is a genus of the Moraceae, the fig family, with very interesting anatomy. Its inflorescences are basically equivalent to an open, flattened fig (or a fig is a Dorstenia inflorescence folded in on itself). The almost microscopic flowers grow in a fleshy structure that is often surrounded by finger-like extensions. After pollination, a seed is produced in a little vesicle and, when ripe, shoots out with considerable force, often landing in pots several feet away. Over the years, I have grown half a dozen different Dorstenia species, but the most frequent volunteers are D. foetida and D. barnimiana. Both species are from east Africa and Arabia. D. foetida grows thick, upright stems with star-shaped inflorescences produced throughout the year. D. barnimiana is a geophyte with a biscuit-shaped underground tuber and deciduous leaves that lie flat on the soil surface. Its inflorescences are more elongated and have fewer extensions than those of D. foetida.
The habit of shooting ripe seeds around the greenhouse is shared by Euphorbia platyclada, a truly bizarre plant from Madagascar. E. platyclada is completely leafless, and its jointed stems look half dead at the best of times. Depending on much light they receive E. platyclada stems can be mottled green, brown, or bright pink. Stems of the latter color resembles coral more than a plant. E. platyclada isn’t as prolific as the Dorstenia species, and I have been very pleased to find a few volunteers.
Instead of shooting seeds, Psilotum nudum (whisk fern) produces tiny spores which drift on the breeze of the greenhouse fans. This is the only greenhouse volunteer that I didn’t originally purchase. The first plant arrived as a stowaway in the pot of a Vachellia cornigera (bullhorn acacia) from a local botanical garden. It has since appeared in several other locations around the greenhouse, but it is so interesting that I don’t begrudge it the space. Psilotum is a genus of primitive fern-like plants that lack true leaves and roots and have a fascinating life-history similar to that of ferns. The sporophyte of P. nudum has a creeping underground rhizome that sprouts green stems tipped with yellowish spore-producing synangia. The spores hatch into a subterranean gametophyte which, when mature, releases eggs and sperm cells. Union of egg and sperm results in a new photosythetic sporophyte.
P. nudum (Matsubaran) has been cultivated in Japan for hundreds of years. See the Primitive Ferns blog for further details on the many cultivars.