We grow quite a lot of species orchids that have a prolonged resting period as part of their growing cycle. As winter approaches we need to start thinking about how to meet the needs of this special group of orchids. In this article I will discuss some of the dendrobiums that we grow (and mostly flower). The genus Dendrobinae has perhaps 1000 species which are divided into sections based on some commonalities in the species grouped together. Our orchid collection includes the following members of the Dendrobium section that are wholly or partly deciduous: Den. anosmum, aphyllum, crepidatum, fimbriatum, loddigesii, nobile (softcanes), pulchellum, regium and moniliforme.
In their natural habitat these orchids respond to environmental triggers such as shorter days, extended dry periods and cooler temperatures with the onset of winter. Their active growing root tips close off and they lose some or all of their leaves to reduce transpiration. With water loss and respiration rates at a minimum these plants can sustain themselves through the cold, dry winter by consuming some of their reserves so the pseudobulbs may start to shrivel. Often their only moisture comes from night mists and dew.
To simulate this you need to gradually reduce your watering and fertiliser regimes. To make managing these plants easier group them together and attached a brightly coloured tag. From about the autumn equinox (Anzac Day is close enough) no further fertiliser is used and watering is tapered off. The leaves start to yellow showing that the plant is reabsorbing whatever minerals it can from the soon-to-be-dropped leaves and once the leaves start to fall watering is stopped completely. This usually happens by early winter and from that time our plants are misted daily in the morning, simulating the morning dew.
In late winter or early spring the plants start to respond to lengthening days and warmer temperatures and the first sign of this is the appearance of flower buds on the almost bare stems. This is NOT the time to start fertilising as it is still the dry season so only an occasional drink of water may be given. As you see the roots become active with fresh, green growing tips you can start increasing water and fertiliser. If needed, this is the time for re-potting. Most of these plants readily form keikis but if you only get keikis and not flowers, perhaps they were not sufficiently chilled or they were watered or fertilised when they should have been kept dormant. Plants should be watered and fertilised copiously during the seven to eight month growing season to build up the food stores ready for the next long drought.
We have a couple of other dendrobiums from a different section that are not deciduous but require exactly the same culture:
Den. chrysotoxum (thanks, Yvonne), lindleyi (aggregatum) and thyrsiflorum. Other deciduous or semi-deciduous orchids that that require similar culture include some catasetums, lycastes, calanthes and habeneria. Knowledge of how the plants grow in nature – their ecology - is a great help to orchid growers.
REF: Orchids Oct.2017; Dendrobium and its Relatives Lavarack, et al.