Undercover: June 2012

Finish harvesting staple crops such as early carrots, potatoes, peas and beans, as their outside counterparts should soon be ready. Use the space for sowing autumn crops next month or fill the gap with a quick-growing summer green manure such as phacelia or buckwheat.

Sowing and planting

Most summer crops should be established by now, but plant out any remaining aubergines, peppers and rooted sweet potato slips early in the month. It’s also not too late to make second sowings of leafy crops such as coriander and basil, and even of quick-growing cucurbits. Planting through black plastic is often recommended for sweet potatoes to increase warmth at the roots, but I find this encourages mice and voles to nibble the shallow tubers and makes it more difficult to get the watering right – I plant them direct into the ground or into large pots instead.

❯ Sow now: Cucumbers, basil, coriander, leafy amaranth.
❯ Plant now: Melons, peppers, aubergines, sweet potatoes.
❯ Harvest now: Courgettes, French beans, sugar peas, broad beans, kohl rabi, summer cabbage, carrots, calabrese, onions, spring onions, new potatoes, beetroot, basil, coriander, strawberries, tomatoes.

Keep in control

Tomatoes often romp away this month – before their energies are absorbed in fruiting – so train and pinch out new growth regularly. Tie each cordon plant to a cane between every leaf joint or twist the main stem round string while the top is still soft and pliable. I find some varieties (e.g. ‘Ferline’) particularly prone to snapping if left too long.

Remove shoots that grow in leaf axils by pinching them out with your fingers, and pull away new shoots coming from the base – these are particularly good at going unobserved and creating a tangle later. The other place unwanted shoots may appear – particularly in non-hybrid tomato varieties – is from the ends of the trusses; use secateurs to cut these away if necessary.

Although bush tomatoes can be left to grow unchecked, I often thin out the branches and remove later flower trusses to allow better airflow and get an earlier crop.

Pollination problems

Crops cosseted under cover nearly always have an advantage over those outside, but sometimes problems with pollination can spoil this. Poor pollination can mean flowers do not set (they drop off without forming a pod, fruit or seed) giving low yields, or it can result in poor quality produce – misshaped squashes or gappy sweetcorn cobs, for example.

The flowers of some crops will self-pollinate – that is, pollen is transferred from the male to female parts within the same flower without any outside help. For many others, however, pollen needs to be carried from flower to flower by insects or the wind. This is where the first problems in greenhouses and tunnels arise, as crops are often grown early, when few insects are about, and doors and vents are often closed so pollinators can’t get in; the atmosphere may also be too still for pollen to move around. In addition, extreme conditions are more likely under cover, making it too hot or dry for successful pollination.

Not all crops are affected, however, and many of the problems with others can be overcome using a few simple tricks.

Encourage pollinating insects If your garden is buzzing with insects, these are more likely to come and go freely into a greenhouse or polytunnel. Grow flowers to encourage them, particularly ones that bloom early in the year – traditional cottage garden plants are good for insects as they have simple open blooms where nectar and pollen are freely available. Grow some inside too, if you have the space – annuals are usually the most convenient for this.

Avoid extremes

An atmosphere which is too hot, cold, or too dry, can adversely affect the pollination of some crops. For example, at high temperatures tomato pollen can be killed (the optimum for pollination of this crop is around 20ºC (68ºF), and squashes may fail to produce male flowers. Heat – particularly at night – also inhibits the set of runner beans, making it difficult to get a good yield of this crop under cover.

Similarly, when the atmosphere is very dry, released tomato pollen may fail to stick to the stigmas, and pollen released from the tassels of sweetcorn may not stick to the silks. It may help to mist the flower trusses of tomatoes with water and to water the ground around sweetcorn an hour or two before tapping the plants.

• Read the full article in Kitchen Garden magazine, June 2012!