Undercover: March 2012

Slower-growing tender crops such as tomatoes and peppers need to be sown in the first half of this month, otherwise they will not fruit in time to get worthwhile yields. In any spare heated space, start off a couple of courgettes or some French beans for an early under cover harvest.

Many of the hardy crops sown in pots and modules last month will be ready to plant out in tunnel or greenhouse borders. Add a row or two of onion sets, planting them closely 2-3cm (about an inch) apart – they can be harvested as fresh green onions before other crops are ready.

Sow now with extra heat

●     For a greenhouse or tunnel: Tomatoes, peppers, chillies, aubergines, courgettes, French beans, cucumbers, half-hardy attractant plants such as French marigolds and cosmos.

●     For the veg plot: tomatoes, celery, celeriac.

Sow now without extra heat

●     Direct in under-cover borders: Carrots, beetroot. In cold areas, a second batch of cutting salads (leaf lettuce, rocket, cress, oriental greens)

●     For the veg plot: summer cabbage and cauliflowers, parsley, onions, leeks, broad beans

Plant out in under-cover borders

●     Lettuce, calabrese, summer cabbage, kohl rabi, sugar peas, spring onions, onion sets

Sort and shuffle the traffic jam of pots and trays on greenhouse benches regularly, otherwise seedlings will become leggy and you won’t spot the odd dry plant, sneaky slug or early aphid (greenfly) attack. Space out pots so that plant leaves do not touch. Those destined for the veg plot can be moved outside during the day to help them harden off. Be ruthless – give extras away or put them on the compost heap.

Overwintered crops

Many overwintered crops will start to bolt this month, although the leaves on the flower stalks of spinach and chard and the flowering shoots of oriental greens are still good to eat, and endives, chicory and parsley slow to run to seed. Cut and use whole plants of spring greens – grown under cover, even their outer leaves are deliciously tender.

●     Harvest: Overwintering chicories, endives and oriental shoots, spinach, chard, parsley, spring cabbage. Spring sown baby leaf salads.

Once these crops are finished, clear the beds and prepare them with compost or well-rotted manure ready for summer plantings. Cover them with a temporary mulch such as black plastic or straw to prevent them drying out.

Provide support for peas and broad beans with pea sticks or canes and string – they still have a lot of extra height to put on before they are ready to harvest.

Frost protectionOn very cold nights, cover propagators with a blanket or bubble polythene to help keep in the heat and protect the vulnerable first shoots of early potatoes.

Also cover flowering strawberry plants with fleece, as the flowers are susceptible to damage by frost.

Start feeding

As the weather warms up, start feeding tender herbs and fruit kept permanently in pots – top dress them with nutrient-rich compost or slow-release fertilizer or give them a regular liquid feed.

With greenhouses and tunnels soon to be planted with new summer crops, the promise of abundant ripe tomatoes is tempting – but do you really need so many? Filling the borders with tomatoes and related crops every year can bring trouble for the future. Root pests and diseases gradually build up and can seriously affect yields (see box at end).

Avoiding such problems is easier than curing them, which is why it is important to try and rotate your crops. However, in a small greenhouse where this isn’t so practical, other measures may be needed.

Crop rotation

In a large polytunnel, crops can be rotated as they would outside on an allotment plot, ideally so those in the same family do not return to the same spot for three or four years. Under cover, vegetables in the Solanaceae family often dominate, as this includes not only the ubiquitous tomato but potatoes and some of the unusual tender crops too. However, restricting these to just one bed can have advantages. You’ll then have room to grow:

●     Alternative tender summer crops such as sweet potatoes or melons with different uses in the kitchen.

●     Early hardy crops such as carrots, beetroot and calabrese, giving you a harvest weeks before those growing outdoors.

●     Autumn and winter crops, such as fennel, spinach, turnips and oriental salads. Space will be ready for them as soon as early hardy crops are cleared and I find this makes all the difference to getting worthwhile harvests.
Crop family check list

●     Tomato family (Solanaceae) crops: Tomatoes, peppers, chillies, aubergines, cape gooseberries, tomatilloes, potatoes

●     Tender crops in other plant families: Early and late French beans, sweet potatoes, yard long beans, chick peas, okra, cucumbers, melons, butternut squash, and more unusual cucurbits such as luffa.

Changing the soil

Unless you forgo tomatoes, a long rotation is usually impossible in a small greenhouse. However, it is possible – if hard work – to change the soil in the borders instead. Every few years, dig out the top soil to at least a 20cm (8in) depth and replace it with a mix of compost and bought-in top soil or good soil from elsewhere in the garden (not the veg patch).

Using green manures

Don’t leave soils to become dry and lifeless between crops; sow a green manure. This encourages beneficial soil microbes which compete for resources with disease microbes, helping to push them out.

Any green manure will help soil health in this way, but if you see definite signs of root problems try using Caliente mustard – available from www.marshalls-seeds.co.uk – a crop specially bred for its disease-suppressing properties. When chopped finely and dug into the soil, it produces isothiocyanate – a natural gas which acts as a ‘biofumigant’ and can reduce diseases such as verticillium wilt and some types of nematode.

Using grafted plants

If your plants are struggling, it could be worth trying ones that have been grafted on to another rootstock. These rootstocks make the plants higher yielding and more vigorous (see KG Nov11), and are also resistant to some root diseases and types of eelworm. You can buy grafted plants of tomatoes, aubergines, peppers, and cucumbers, but in a limited number of varieties. If you can’t get your favourite tomato as a grafted plant, you could buy seeds of a rootstock (e.g. from Moles Seeds www.molesseeds.co.uk) and try grafting it yourself – it is fiddly, but not specialist stuff.

Container growing

Sometimes growing plants in some kind of container is the best option, although compared to plants in a border they are usually more difficult to keep correctly watered and fed. You could use:

●     Growing-bags – These are particularly susceptible to both drought and waterlogging. It may help to plant into bottomless pots stood on top of the bags themselves, or use proprietary ‘watering pots’ that contain a reservoir.

●     Other containers – The bigger the volume of compost available to plant roots the better. For tomatoes, I use at least a 30cm (12in) diameter pot for each plant; slightly smaller ones for ‘patio’ varieties of peppers and aubergines. A whole range of proprietary planters are now available – including 30cm (12in) deep transportable greenhouse beds, which can be folded away when not in use, try www.marshalls-seeds.co.uk Potatoes do well simply in compost bags.

●     Ring culture – Here plants are grown in bottomless pots resting on a bed filled at least 15cm (6in) deep with porous aggregate (such as gravel). The gravel provides a large reservoir of moisture for the plant, yet it can be isolated from the soil, and cleaned and re-used each year. Tomatoes, with their strong roots, particularly benefit from this system.

●     Straw bales – Straw bales hold a lot of moisture and as they break down provide warmth, nutrients and carbon dioxide to help plant growth. You need small bales of straw that have not been treated with chemical ‘straw shorteners’ or hormone weed killers. Put them in place several weeks before planting and soak them with water and nitrogen-based feed to start them decomposing, and then set out the plants into small holes filled with potting compost. I have good results with tomatoes grown in this way.