It is perfectly possible to grow many herbs without any form of winter protection or artificial heat. But in colder regions a small, frost-free greenhouse is a great help in extending both the season of growth and the range of plants it is possible to grow.
It is often easier to raise plants from seed sown in trays. Under controlled conditions, the success rate is usually higher. With a greenhouse you can start sowing much earlier than if you had to wait for the right outdoor conditions. But remember that young plants, grown on from seedlings, must be acclimatized gradually to being outside, before they are finally planted in the garden. Do this by standing them outside in the daytime for a short period, or transferring them to a cold frame.
Parsley is a good candidate for sowing under glass. Although it is reasonably hardy, it needs heat to germinate (about 18ºC, 65ºF), which is why it takes so long to emerge when planted straight into the garden early in the year before the soil has warmed up. Germination will be much quicker and more reliable if the seeds are started in trays in the greenhouse, with a view to transplanting outside once grown.
Basil is almost impossible to raise from seed in temperate regions without the benefit of glass. But it is not difficult to get good results if you follow a few guidelines:
- Do not start too early in the year; allow spring to get well under way first, when it will be easier to supply a temperature of 15–18ºC (60–65ºF).
- Scatter seeds as sparsely as possible, so that little thinning-out is required later and root disturbance minimized.
- Provide the seedlings with adequate ventilation and do not overwater to reduce risk of “damping-off” disease.
- Grow plants on in a large container, rather than planting them directly into the soil. They can then be kept outside or moved into a greenhouse, according to current weather conditions.
Unless the summer is exceptionally
cool and wet, sweet basil (Ocimum basilicum) will usually grow well in a pot outside, but the purple-leafed kinds seldom reach their full potential unless kept under glass.
Chilli peppers (Capsicum spp.) also need to be grown in the greenhouse in cooler climates if they are to produce mature, ripe fruits. In parts of the United States chilli peppers can be grown outside. Other herbs to try are the popular Japanese salad plants known as shiso (Perilla frutescens and P. frutescens ‘Crispa’) and the culinary flavouring plant, lemon grass, much used in Thai cookery (Cymbopogon citratus).
Adequate shading and copious watering are necessary for plants grown under glass in the summer.
For container-grown plants that are not fully frost-hardy, such as bay, lemon verbena and myrtle, as well as more tender subjects such as scented pelar-goniums, a cold greenhouse, provided it is frost-free, gives enough protection to keep them alive through the winter. In the summer months they can stand outside in the garden, as long as they are moved under cover before the first winter frosts.
Many of these plants will lose their leaves, even if they remain evergreen in warm climates. Once they have been moved into the greenhouse in the autumn, cut them back or trim lightly, according to individual requirements. Over the succeeding winter months give them a minimal amount of water and no fertilizer to ensure dormancy.