Resting Plants in the Greenhouse over Winter

For very many plants, a winter rest is essential for their continued development and success but, in the greenhouse, some plants that would naturally be growing almost all year round can also be kept dormant over the winter to economize on heating and lighting. How you treat plants as they approach their winter rest is extremely important. So, too, is the way you treat them over the winter, and how you care for storage organs like bulbs, corms, tubers and rhizomes.

Many tender plants can survive cold winters in an unheated greenhouse. For instance, many of the summer-flowering bulbs use this cool period to develop embryo flowers and leaves while appearing dormant externally. Others (like tender herbaceous plants and exotic greenhouse perennials), if kept in a dry environment, slow their growth rate right down until they are ready to flourish again in the spring.

Plants with storage organs

Plants that overwinter in the form of storage organs include all the summer-flowering bulbs, tubers such as gloxinia, begonia, and gloriosa, and rhizomes like achimenes, smithiantha (temple bells) and canna. A critical time for these comes after the flowers fade; they then begin to store the foods needed for strong growth in the following year.

In the case of bulbs the entire flower in embryo form is developed inside the bulb, starting after flowering and continuing to form until the foliage begins to wither. So at this time, care with regard to feeding and watering will reward you with a good performance after the plant is started into growth again the following year. Do not chop the foliage off bulbs or tie the leaves in knots as soon as the flowers have faded. Do all you can to keep growth luxuriant and healthy; give frequent liquid feeds, foliar feeds and pesticide sprays to keep the foliage free from pests and to build up the storage organ so that it will grow well the following year. Only when the foliage begins to die down naturally should you gradually reduce watering and feeding. In most cases you can allow the soil in the pots to dry out slowly. Then turn the pots on their sides and store the plants (dry) in this manner in a frost-free place for the winter period.

With some plants (such as tuberous-rooted begonia and gloxinia) it is better to remove the storage organs and store them in clean, dry sand after removing any adhering potting compost, dead roots and foliage. Drying out as a winter rest, however, is not a general rule. There are cases where storage organs are better kept slowly growing. A typical example is hippeastrum (often wrongly called amaryllis). This is an evergreen bulbous plant that will produce far better blooms if you give it just enough warmth and moisture to maintain the foliage; 5-7°C (40-45°F) is an adequate temperature. Leave plants with brittle tubers or rhizomes (like gloriosa and achimenes) undisturbed, and store them in pots of dry soil on their sides. These plants will have multiplied considerably during the growing season. Wait until it is time to restart them into growth in the spring before splitting or dividing them. If you damage them then they are much more likely to heal quickly, but if the organs are damaged just before the winter rest period there is a greater risk of rot setting in.

Protecting with fungicides

Fumigation protects against grey mould, a common infection in winterTo help reduce the likelihood of rot or fungal attack, it often helps to dip the storage organs in a solution of benomyl and allow them to dry thoroughly before storing. Fungicidal dusts (used according to maker’s instructions) can also be beneficial and there are several suitable proprietary products on the market.

Ripening before storing

Some plants, particularly those with bulbs or corms, may not flower well unless they undergo a short ripening process before storage for the winter. Typical of these is polianthes (tuberose); it likes to be given a ‘sun bath’ so expose it to as much sunshine as the late summer and early autumn (July and August) will provide before immersing it in dry sand for the winter.

Place dahlia tubers in the greenhouse (with their stems pointing downwards) for a few weeks, to allow all sap to drain before storing in dry peat or sand. Cut back chrysanthemum growth, lift the plants, free the roots from soil and then store them in trays of sterilized potting compost until it is time to restart them in late spring or early summer (April—May).

Herbaceous and pot plants

When herbaceous plants and perennial pot plants in the greenhouse begin to slow down or reach dormancy in autumn, you can prune them back — not too severely —so that they will not take up so much storage room. Most plants with ordinary roots should not be allowed to go completely dry over the winter but, on the other hand, never let them get too moist. Much also depends on winter temperature, and where this is very low — perhaps approaching freezing — keep the plants almost dry. Wet, cold conditions are certain to cause root rot. Tender plants like greenhouse fuchsia, pelargonium and even the more exotic subjects like strelitzia, Nerium oleander, maranta (prayer plant) and aphelandra, will usually survive a very cold winter if kept on the dry side. Some, like maranta and other foliage plants, may come through the cold of winter looking the worse for wear, but new growth rapidly occurs in spring when you recommence watering. Shabby growth can then be removed.

If you grow standard fuchsias, take great care not to rest them to the extent that they die back. Although growth will resume in spring the head could fail and new shoots come from the lower stem.

When whips (the supporting stems of standards) are being grown on from rooted cuttings, they must not be allowed to rest too much. Keep them growing slowly by maintaining adequate temperature and moisture. This applies to all plants grown as standards including abutilon, pelargonium and chrysanthemum (marguerite).

Greenhouse atmosphere in winter

The greenhouse atmosphere is also important to the well-being of resting and overwintering plants. It is vital to avoid excessive humidity, so provide plenty of ventilation whenever weather permits. Home greenhouse owners tend to be afraid to ventilate. Winters in Britain are rarely freezing for long periods and often there are very mild spells. Since most people grow plants needing little more than frost-free conditions there is no reason why ventilation is not possible much of the time.

Special care is needed if you heat the greenhouse with paraffin heaters. Oil produces about its own volume of water on combustion. This leads to much condensation and a very wet atmosphere unless there is adequate ventilation. In such conditions dormant plants are very prone to fungal troubles. The main enemy is grey mould (botrytis) that attacks both dead and living vegetable matter.

It forms a greyish-brown furry mould that, when disturbed, distributes clouds of dust-like spores and so spread the fungus to other plants.

If your paraffin heater is fitted with a humidity trough, do not fill it with water in winter and, of course, never damp down the greenhouse. To help protect resting plants from grey mould in winter a wise routine measure is to fumigate (on a still day) with TCNB smokes. In winter this is better than the use of wet sprays, since it avoids raising humidity.

If you have an automatic watering system, allow it to go dry, or shut it off, for the winter months and water by hand. This will lessen the danger of plants — and air — becoming too moist.

13. July 2011 by Dave Pinkney
Categories: Greenhouse Gardening, Resting Plants | Tags: , , | Comments Off on Resting Plants in the Greenhouse over Winter


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