Hardy orchids in the greenhouse

This is how I grow orchids in pots, but other people use different approaches that work just as well or better! Some basic “rules-of-thumb” that apply to most hardy terrestrial orchids can be summarised thus:

A partially shaded position is usually best
Compost must be moisture retentive but free-draining
Compost must be quite low in fertility with little or no extra feeding
Many prefer an alkaline compost
Though hardy in the ground, freezing of pots will kill the plants
Avoid extreme conditions (temperature, watering, fertility, light etc)
Ensure good ventilation – avoid high humidity

Potting Up

Clay pots for potting up look nice but are not essential and they tend to dry out quickly. They do work well in a sand plunge, because of their porous nature. I use plastic pots mostly and they work perfectly well.

A staggering array of different compost recipes have been recommended for hardy orchids, some far more complicated than is really necessary. Generally they like to be in moist but well-drained soil of quite low fertility. It should also be “loose-structured” to aid oxygenation of the roots. Dense, claggy or sticky soil should be avoided. Many need alkaline conditions, but some prefer neutral or mildly acidic soil.

The simplest mix is 50% mineral component for good drainage and 50% organic component that is humus-rich for moisture retention and has some nutrients. The mineral components I have used are: horticultural sand, fine horticultural grit, fine limestone chips, fine chalk rubble, and perlite. They can be used alone or in combination, but it’s certainly not necessary to use them all together! I typically use equal amounts of limestone chips and sand to give gritty drainage and alkalinity. For the organic component I often use peat-free multipurpose compost and composted bark in equal amounts. Orchid bark works well but is too expensive for bulk use. Commercial loam-based composts (eg John Innes 1 or 2) can be useful but they contain some peat usually. Garden leaf mould can also be useful. Check whether the orchid prefers alkaline, neutral, or acid conditions in nature and adjust the mineral component accordingly.

The ratio of mineral to organic component can be varied to suit the orchid, eg 60% and 40% or vice versa. A small amount of bone meal can be added to the mix, to help root growth. Be careful not to over-do it though; certainly no more than a level teaspoon in a large pot of compost. Avoid fertilisers high in nitrogen, as this can kill off orchid tubers.

Plant the tuber with the bud uppermost and not too deep below the surface; about 2 to 4cm is usually fine. Often the tuber sinks lower year on year anyway. I usually dress the top with a thin layer of horticultural grit to help avoid neck-rot and because it looks nice, but it isn’t strictly necessary. Don’t forget to label the pot.

Plant Care

The watering regime is something that comes with practice. Most orchids are more resistant to under-watering than they are to over-watering or water logging, which will be fatal. However don’t ever let the soil dry out completely (even during dormancy) as this can also be fatal. I usually water each pot only until the whole surface is wetted, so not very much at a time. In late summer and autumn this can be maybe once per week but probably twice or more per week in spring and summer when growth is fast or when the temperature is high. Don’t let the pots sit in a waterlogged tray. In winter, water only to keep just moist, and not at all when the temperature is near freezing. Rain water is considered best, but I often do use tap water. In a chalky, hard water area this is usually OK but may not suit acid-loving and some other species.

Light, temperature, and air are very important factors in greenhouse-growing. Hardy orchids need to have the natural outdoor cycle of colder night and warmer day temperatures through the changing seasons. They don’t like to sit in still, humid air as this can cause diseases and rot. Nor do they like to bake in temperatures much above about 25C. While many grow in full sun in nature, many like to have a bit of shade too.

For these reasons I put up greenhouse shade netting when the temperature rises in spring, but take it down in autumn so that wintergreen species can get maximum light. I put bubble wrap insulation below the greenhouse roof in winter, but would only use a heater for prolonged below-freezing temperatures. If a pot freezes solid for any length of time the orchid tuber will die, but otherwise they benefit from being cold in winter. For ventilation have windows open all the time, except in freezing weather.

One problem with the greenhouse I have recently become more aware of is to avoid allowing pots to warm up too quickly in late winter and early spring. With climate change and milder winters the risk is increasing. This sends the wrong signals and seems to trigger the plants into early dormancy, such that they fail to flower and growth can be badly affected.

For ongoing maintenance, I try to re-pot with fresh compost regularly as the structure tends to gradually break down. Do this every 2 years, or yearly if you have time. Thus nutrient levels are replenished and the tuber depth adjusted if needed. Instead of re-potting I sometimes top-dress the pot around the edges with a very small amount of bone meal mixed with damp sand. Be aware that, when dormant, some of the small, round Orchis or Ophrys tubers are very hard to find in compost; they have no roots at this time and are hard to distinguish from a large piece of grit! This is also true of A. morio, for example, and it is best to only re-pot at times when you can locate them via their foliage. Orchids are fairly amenable to re-potting at any time of year, but I wouldn’t disturb them in the run up to flowering, when it is better to just enjoy them. Here are some notes on growing particular orchid groups:

Dactylorhiza

The Dactylorhiza or “Marsh orchid” species are mostly easy to grow. They often favour fens and very wet areas in the wild, but in a pot they only need moist soil. Poor drainage must be avoided. A suitable planting mix is about 60 to 70% organic matter for good moisture retention, opened up with about 30 to 40 % mineral components.

These are “winter dormant” species .The shoot appears in early spring giving a low rosette of leaves from which a spike emerges with flowering in June. Thereafter top growth dies back in summer, while underground the old tuber dies but a new tuber has already formed for next year. This is like a miniature parsnip but with multiple lobes or fingers pointing down. There are several long wiry roots, so use a deep pot, eg 15cm or so. A single large bud at the top of the tuber gives rise to next year’s shoot. If growing well more than one replacement tuber may form, thus causing vegetative increase into clumps.

The more commonly available Dactylorhiza plants in the UK include D. fuchsii, D. maculata, D. praetermissa, and possibly D. incarnata and D. purpurella. In addition there are several European species that are often available and rewarding to grow; these include D. foliosa, D. majalis, D. elata and others. Also there are several good Dactylorhiza hybrids that have large flower spikes and can be good clump-formers. D. x grandis is a naturally occurring hybrid between D. fuchsii and D. praetermissa. Also particularly good are the “Blackthorn” and “Deep Purple” hybrids.

Ophrys, Orchis, Anacamptis & Serapias

These genera are “winter green” plants, ie the foliage is present in autumn and winter as a low rosette of leaves. This grows and shoots in spring, flowering between April and July. The top growth dies off in summer, leaving a dormant replacement tuber. They are round, oval, or turnip-shaped with no roots, and usually quite small, eg only 1 to 3 cm in diameter. The bud is initially tiny then grows to form the shoot. Fairly short, wiry roots develop at the base of the shoot.

I use a slightly higher ratio of mineral component in the compost for good drainage: eg 60% mineral plus 40% organic. The main difference with these species is that the tubers are adapted to being dormant and almost dry in the sun-baked ground of summer. Some like to harvest the tubers at this time and store them in a cool, dry envelope until autumn so they don’t accidentally get over-watered and rot. I don’t usually bother with this. Firstly, because they are fine in their pots so long as you water just enough to prevent complete dryness. Secondly, it is very difficult to find the smaller tubers. Thirdly, one often forgets to re-plant them in time and the shoot has started growing! The species that are more commonly available include O. apifera and various European Ophrys species, O. mascula, A. morio, A. laxiflora, A. pyramidalis, and S. lingua.

Epipactis

These can mostly be grown similarly to Dactylorhiza. They are “winter dormant”, ie the shoots appear in spring. Flowering occurs in June, July, or August depending on the species. Afterwards the top growth dies back. Instead of a tuber there is a thin, stick-like rhizome system, which is branched and has many long roots. One or several buds form on the rhizome and remain dormant over autumn and winter. They then give rise to the new stems in spring, so that a mature plant can form an impressive clump of flower spikes. Use a large pot so there is space for the rhizome to expand. The same compost mix can be used as for Dactylorhiza. Examples of the genus that can often be available are: E. palustris, E. gigantea, and E. veratrifolia.

Cypripedium

Here are some very brief notes on growing the species and hybrids of Slipper orchids. This is a specialist subject in its own right and much detailed advice is available elsewhere. They are typically woodland-edge orchids with some similarities to Epipactis and preferring partial shade sheltered from drying winds. Around 60 species are known from temperate regions of the northern hemisphere, and many artificial hybrids have been created. The hybrids are often more vigorous and easier to grow than the species. A branched rhizome spreads horizontally just below the soil surface, with many roots growing downwards. They are “winter dormant”, the shoots appearing in early spring and flowering in May or June. Plants increase vegetatively if growing well, forming spectacular clumps with large flowers.

Gritty, free-draining compost should be used, with around 60% or more mineral component. The rhizome should be planted with the buds only just below or even at the surface. Hybrids can benefit from a little more nutrient than most orchids, even a dilute liquid feed occasionally. The seasonal watering regime is as for Dactylorhiza, never letting the root system dry out completely. Particularly important, though, is to minimise winter wet. Otherwise they are very frost hardy. Protect them from full sunlight. There are too many species and hybrids to go into any detail, but C. reginae is attractive, readily available and quite easy to grow. C. calceolus, the Lady’s slipper, holds a great attraction because of its scarcity here in the UK., but plants of European origin can be obtained.

Other Genera

Without going into any detail, there are various other genera of hardy orchids from around the world that are obtainable and can be successfully greenhouse-grown. These include, but are not limited to, Bletilla, Pleione, Calanthe, and Cymbidium which are mostly from East and South East Asia. Also of note are Caladenia from Australia and Disa from South Africa.

Hardy Orchids as houseplants

Hardy orchid lovers without easy access to outdoor growing facilities such as a greenhouse or a garden might want to try growing them as houseplants. The “tropical” epiphytic orchids are, after all, successfully grown this way. The main challenge would be temperature control. Outdoor seasonal and day to night temperature changes are more important for the hardy orchid annual cycle than they are for “tropical” orchids. These variations are of course very much reduced indoors.

Nevertheless on a small scale this could probably be made to work. Most growing conditions such as light, compost, watering etc can be met in exactly the same way as with greenhouse growing. It would, though, be necessary to give the pots a decent cold winter of at least 2 months by locating them in an unheated area, or somewhere fairly dry and sheltered outdoors. Also a south facing window sill should be avoided, particularly in spring and summer, as the compost can rapidly overheat and cause early dormancy or worse.