Undercover: January 2011

January can bring severe weather, and last year was a test for polytunnels – a reminder that they might have to cope with heavy snow and hard frost. However, such dormant spells also give us time to prepare for the season ahead.

Sow

Tomatoes, peppers, aubergines – but only if you have heated space to grow plants on.

Lettuce, spring onions, spinach – in a heated propagator for planting out in undercover beds.

Harvest

Endives, chicories, oriental greens, lamb’s lettuce, claytonia, landcress, spring onions, spinach, chard.

Beating winter weather

Remove any lying snow from tunnel roofs and sides – don’t let it accumulate. Although a light layer provides good insulation against frost, snow is heavy stuff and can bend or buckle the tunnel framework. Gently sweep it off using a soft broom or a DIY sweeper – wrap any sharp edges with rags to cushion them. One compensation of this extra job is that the shedding snow often brings off green algae with it, leaving the polythene sparkling clean.

Last year the snowfall was so heavy that many gardeners resorted to emergency DIY props and braces to prevent tunnel damage, but there are permanent ways of strengthening the structure. You can add ‘crop bars’, for example, which span the width of the hoops above head height. They are useful for supporting plants, but are also said to increase the weight that the tunnel can support by 20 per cent, making it more resistant to wind and snow. Although they should ideally be put on when you build or re-cover the tunnel, you can add them later and many manufacturers sell them separately. First Tunnels, for example, supply a wide range of spare parts and tubes to your specification, so you can even get some to fit old secondhand tunnels.

When frosts are severe, cover undercover crops with winter weight (30g) fleece or a double layer of light fleece. It can be left on during the day for short periods if temperatures are low. Greenhouse and polytunnel doors can be frozen shut – don’t try to force them or damage may result.

Get ready for sowing

Unless you live in a very mild area or have a heated greenhouse, you will usually gain little from January sowings. Time is better spent preparing for the spring rush:

• Make sure you have all the seeds you need.

• Clean pots and seed trays. Scrub them when dry with a stiff brush to remove caked soil and compost – an old toothbrush helps with small pots, but for larger ones you can’t beat the old-fashioned round pot brushes (still available, eg from Harrod Horticultural www.harrodhorticultural.com). Sometimes this dry scrubbing is enough, but washing them as well will help to prevent disease spread. Put them in warm water with horticultural disinfectant and leave to soak for several days – there is then more chance that spores are killed, particularly in cold weather. Clay pots especially benefit from soaking.

• Buy new bags of seed or multi-purpose compost (or make up new batches of your own mixture). Use up those from last year on beds or borders, or mix them with other ingredients to fill large troughs or pots.

Potatoes

Buy seed potatoes for next month’s undercover plantings and put them in a warmish light place (about 10C/50F) to chit. The earliest varieties make best use of undercover space – ‘Rocket’ is said to be one of the first to produce harvestable tubers, but I find it rather tasteless. My favourites are ‘Dunluce’, ‘Orla’ and ‘Lady Christl’.

Undercover fruit

Bring in pots of strawberries that have been outside or in cold frames – they should have had sufficient chilling by now to make them flower well. Remove any dead leaves and stems, and make sure the compost is thoroughly moist. Space the plants out along the edge of greenhouse staging or on a narrow shelf so the fruit can trail. This helps keep it clean and disease-free, and makes it easy to pick.

Small greenhouse tips

Soil-borne diseases can be a particular problem in the borders of a small greenhouse because it is difficult to have a proper crop rotation. If your tomatoes have lost vigour over the last few years, one of the following options might help this summer’s crop.

• Order grafted plants – these are grown on rootstocks which themselves have good resistance to many common soil-borne diseases, and the extra vigour they give to the plants helps them cope better with other problems. Grafted tomatoes, peppers and aubergines are all available (eg from Suttons, www.suttons.co.uk). The disadvantage is that it limits your choice of varieties.

• Plan to grow plants in grow-bags or large pots, or by ‘ring culture (where bottomless pots rest on an aggregate such as gravel); then the plant roots have no contact with infected soil. The disadvantage is that the plants need more watering and feeding than those grown directly in the border.

• Dig out the soil down to the subsoil and replace it with a mix of good disease-free soil and compost or well-rotted manure. The disadvantage is the hard work, and unless you are digging elsewhere in the garden – a pond or foundations for a path, for example – you will probably need to buy in topsoil.