The short answer is anywhere there is sufficient light! However this leads to another question. What is sufficient light? When plants are completely dormant and the compost has dried out completely, cacti and other succulent plants need less light and could be moved to a position with lower light levels if necessary to achieve an appropriate temperature level. This applies to all those cacti and other succulent plants that grow in spring and summer, and then stop growing in autumn as the days shorten, though there are some winter-growing exceptions discussed on other pages.
When growing actively, plants need very good light. This ensures that no etiolation (spindly growth) occurs. Many garden centres are unaware of the needs of cacti, and you will see plants with pale thin growth on top of normal bodies. Do NOT buy these unless they are very cheap and you are prepared to spend some time and effort chopping off the spindly growth and waiting for offsets to develop below that can then be rooted down. (Fig. 1).
Normally a windowsill with a southerly aspect will provide an adequate light level. Given some ventilation and carefully controlled watering (again watch for any etiolation), this will provide a growing space for plants with lower light-level needs, such as Haworthia and Gasteria, and smaller-growing cacti such as Rebutia, Gymnocalycium and some Mammillaria (Fig. 2). However it will be difficult to grow more vigorous plants with higher light demands satisfactorily in such conditions.
Your next option is a cold fame. Site your frame in the sunniest position available in your garden, where the fewest shadows fall. The temperature in a closed frame rises rapidly when the sun hits it, so ventilation is vital. In winter, the frame will keep your plants dry, but not warm unless you use a form of heating (see later). Because of the better light, the range of plants you can grow increases. (Fig. 3).
The best option, when money and space are available, is a glasshouse! Flat sites are easiest, but the site can be levelled or built up where needed. If possible, put in a concrete base or paving slabs with a damp-proof membrane below to stop damp rising from the ground in winter. Choose the sunniest position you can, and buy as large a glasshouse as you can afford that fits your site. You do not have to fill it. Growth will absorb your spare space in time. Almost everyone who collects for several years complains about their lack of space. Another advantage to a larger glasshouse is that it maintains a more even temperature, without the extremes met in, for instance, a small frame (Fig. 4).
Conservatories can also provide a good environment for growing cacti and other succulents, particularly if they are in a sunny position and some ventilation can be provided on hot days (Fig. 5). However if children are likely to use the conservatory, avoid any spiky plants or put them well out of reach.
How do I provide heat?
First you need to ask yourself whether you want to grow plants which need heat. There are many types of cacti and other succulents which go dormant for the winter and, if completely dry, will tolerate unheated conditions even with temperatures well below freezing, though some insulation would be advisable (see below). Suitable plants can be found amongst the rebutias and some Echinocereus, Agave, Aloe, Gymnocalycium species, and also some from the Mesembryanthemum family.
Heating
If you plan to install heating, you have a choice of several ways. Ruling out the solid fuel boilers of old, your fuel, most likely will be electricity, or perhaps gas (bottled or mains), oil or even paraffin.
Electricity
This is the cleanest, the most controllable and the most popular fuel. You might find advantage in using one of the special night-time tariffs offered by some suppliers. Your supply has to be run out to the usage point by a qualified electrician and this can be expensive. All your electrical apparatus and fittings must for safety reasons be waterproof. Special fittings are available but at a cost.
Ensure that the maximum total power consumption of your heating device within the greenhouse does not exceed the maximum power supply available. Use the manufacturers’ recommendation for the power rating required to the size of glasshouse you have erected. Ideally your heater should only be heating for half the time under normal low outside temperatures of about zero degrees Celsius. The rating you need depends on the temperature you want to maintain under these conditions. Roughly, for every five degrees Celsius above the outside temperature, you need to double your capacity.
If possible choose a heater with a remote thermostat. Those built into the apparatus tend to pick up heat by conduction from the elements and do not reflect the air temperature in the enclosure you are heating. Fan heaters give a good dispersal of the heat and a buoyant atmosphere, minimising damp areas. Tubular heaters are perfectly good particularly for a cold frame, if placed at a low level as heat rises. Such heaters may also cope with low temperature rises in larger enclosures, but do not give the circulation of a fan heater.
Gas
A range of heaters using this more convenient fuel has been developed. You need to have the supply run out to the glasshouse or cold frame, or use the bottled variety. Modern heaters are thermostatically controlled and clean. Some claim that the extra carbon dioxide they produce improves the growth of their plants, though they also produce moisture, which is not wanted. It is often the cheapest fuel in use, although it needs some capital investment initially and later in the life of the heater for replacements.
Oil
Just a brief mention of fuel oil must be made. There is no doubt the installation costs are high, but many nurseries use this form of heating cost-effectively, in spite of big price fluctuations, for their very large glasshouses.
Paraffin
This has to be fetched and stored. It has increased in price significantly over the years. Heaters are obtainable from garden centres in a range of capacities. Some are suitable for the cold frame, whilst others will provide adequate heat for a small glasshouse. These heaters may ‘smoke’ and deposit soot on the plants if not kept clean and adjusted, and some ventilation is advised. They do not have thermostatic controls, relying on adjusting the heat level by turning the wick up or down. They also need regular attention to turn them on and off and refill them.
Insulation
Because heating costs are high, insulation is advisable. The best form is bubble polythene, preferably the ultraviolet-light-stabilised type available in garden centres. This can be fastened inside the roof and sides of the frame or greenhouse to retain heat and keep out the cold. It is particularly advisable for unheated frames and greenhouses. Many growers keep it on all year round where the plants are in a very sunny location, so that it can provide some light-filtering to prevent scorch from extreme heat build-up. Examples of simpler forms of insulation are newspapers, horticultural fleece etc placed over the plants at night and removed in the daytime.
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