A greenhouse is the ideal laboratory in which to raise new plants – from seed, cuttings, divisions and offsets.
A small unheated greenhouse can provide ideal conditions for rooting cuttings, germinating seeds and carrying out most other types of plant propagation. If you can afford to install some form of heating — even if it is only a small heated frame — within the greenhouse, the scope for raising new plants is even wider.
In order to germinate or root well, most seeds or pieces of plant material taken from indoor plants and soft-stemmed garden plants need warmth. And although many hardy, woody garden plants will germinate or root at lower temperatures — easily provided in a cold frame or outdoor nursery bed — a constant, controlled greenhouse environment will ensure much better results.
It is usually possible to keep plant material suitably warm and moist in an inexpensive plastic-topped propagating case or improvised unit placed on an indoor window-sill. But space is invariably a limiting factor indoors, so if you want to propagate large numbers of plants — especially if you enjoy growing annual bedding plants —you will need to set aside a part of the greenhouse especially for this purpose.
Another problem associated with raising new plants indoors is lack of light. Although a windowsill may be very bright, the light comes from one direction only and seedlings in particular are drawn towards it, producing leggy, thin-stemmed plants. Devices can be set up to reflect light back on to the young plants — using aluminium foil reflecting screens, for instance — but such methods are cumbersome and unsightly.
Heat passing through glass when the sun strikes it is a real problem with plants raised on window-sills. If seedlings and cuttings are not shaded from direct sun they soon dry out, wilt and die.
A greenhouse eliminates these problems. If properly sited, natural sunshine keeps the plants well lit throughout the day and from all angles, so new growth is even, compact and upright.
The air temperature increase caused by sunlight through the greenhouse glass or plastic cladding is used to beneficial effect. With better air circulation and the means to maintain much higher humidity, high temperatures no longer dry out the soil and kill the plants. Instead, high and constant temperatures encourage better plant metabolism and hence improved, rooting and subsequent growth.
Taking cuttings or sowing seeds and merely placing the trays or pots in a greenhouse will not ensure success — you must get all the growing conditions right and make appropriate allowances for different types of plants.
A humid atmosphere is absolutely vital for vegetative propagation — using cut sections of shoot, stem or leaf — of most varieties of plants. Humidity will help to prevent water loss from a cutting, which is temporarily unable to take up moisture readily because it has been severed from its roots.
Loss of moisture is greatest where leaves are included in the cutting material, because the leaves continue to transpire — give off moisture from their pores — as long as they live.
Leafless stem-section cuttings, on the other hand, don’t transpire very much, or not all. But if the rooting compost in which they are planted dries out, they will soon wither and eventually die.
Maintaining high humidity in the air is more effective than watering as a means of maintaining the optimum amount of moisture in the rooting compost. This is because watering — even if carried out on a very regular basis — is intermittent and so there are fluctuations in the water content of the compost. Where frequent regular watering is not possible, the compost can go through severe fluctuations of wetness and dryness, doing great damage to the emerging roots or discouraging rooting in the first place.
However, humid air is not a requirement for the rooting of cuttings taken from fleshy-stemmed plants, such as cacti and other succulents and certain perennials and sub-shrubs, including pelargoniums. In fact, humid air can encourage the growth of fungal spores, resulting in various rot diseases.
Even where high humidity is required, the secondary effect of encouraging fungal diseases can be a major problem unless good ventilation is maintained. Always ensure that fresh air circulates around cuttings or seedlings, at least from time to time.
Most cuttings and seeds start growth best at a temperature of at least 18°C (64°F). Tropical plants generally require higher temperatures — above 24°C (75°F). Use a minimum-maximum thermometer to monitor the day and night temperatures and take steps to ensure that the heating remains as constant as possible.
Rooting powder contains a synthetic plant rooting hormone, identical to the one normally possessed by plants. Soft-stemmed plants usually root swiftly without the assistance of extra hormones, and a thick coating of powder on the base of the stem or leaf can actually form a barrier against successful rooting. If you want to use a rooting powder on soft-stemmed cuttings, make sure the dusting is light.
Woody stems root more slowly and the process can be speeded up by dusting the base of the cutting with hormone rooting powder. Buy fresh powder every year — it becomes less effective with age.
The usual common garden potting composts available from most nurseries and garden centres, are not suitable for either seeds or cuttings. The texture of these mixtures is much too heavy and their nutrient content is too high, and this can scorch the young delicate roots. All that is required of the compost is that it holds moisture well, yet prevents waterlogging.
Use a proprietary seedling/cuttings compost or a general purpose compost. Several are readily available, including organic composts, multi-purpose or coir mixtures which are recommended for sowing seeds and rooting cuttings.
All rooting mixtures must be sterile — the presence of fungal spores leads to damping-off and other rot diseases. Use compost taken directly from a sealed bag. Never use garden soil or old compost, however good it may look.
Germinated seeds and rooted cuttings soon need more food than is provided by seedling/cuttings composts. They should be transplanted into potting compost, which contains fertilizer, as soon as they are large enough to handle, although liquid feeding with a weak solution of ordinary house plant fertilizer provides a good temporary alternative if transplanting has to be delayed for a short while.