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Under cover

May is often the nicest month of the year. There is heat in the sun, night frosts become a thing of the past and the days are long. Everything grows with an enthusiasm that is hard to match. In fact, if you photograph the greenhouse, or polytunnel, at the beginning of May and then again at the end, it can be difficult to believe that such a difference is possible in the space of one month. Those tomatoes and cucumbers will be heading upwards, and the wide spacings of planting time are rapidly filling up with leaf, flower and fruit.

A summer mantra

Ventilate, water! Water, ventilate! If you don’t get these two factors right for the next few months, then you may as well wave goodbye to good crops under cover.
May daytime temperatures can be high enough to kill plants in small pots and even large plants, with roots in the earth, can wilt if temperatures under cover reach the high thirties.
The beginning of the month can still bring cold nights, so shut doors and windows in the evening and open them in the morning to avoid dramatic temperature swings.
Correct watering is crucial to healthy growth. Keep borders damp and wet down paths if it is really hot. Don’t over-water small plants in pots.

Indoor cucumbers

If these were sown in February, or early March, they could be 45cm (18in) tall and covered in small fruit in early May. Cucumbers like a free-draining root run, but they need moisture as well. If plants have been raised in a propagator, they also need a bit of extra warmth at this time of year. The ideal system is to make a hot bed. This can be as simple as a pile of fresh, strawy manure, covered with soil or compost. As the manure breaks down it releases heat, which warms the soil and keeps roots at a steady temperature. Wooden sides will keep the bed in shape and hold in a bit more heat as well.
Allow the extreme heat of a newly constructed bed to disperse for a few days before planting cucumbers out 45cm (18in) apart. You can get away with much earlier planting in this way. If a cold night is forecast, simply throw some fleece over the top of the plants.
With a little care and attention, you could well be picking your first ripe cucumbers this month.

Melons, aubergines, peppers

These seem to be the plants that suffer most if temperatures fall too low at this time of year. If the weather is settled and mild and night-time temperatures in the greenhouse don’t fall below 12-15C (53-60F), then you can plant these tender plants in the border soil, or into growing-bags, or large pots. If there is any doubt about the weather, it is safest to keep the young plants growing with a little added heat. Pot on into larger pots if necessary and plant out in June if that is what the weather dictates.

Tomatoes

Most greenhouse tomatoes are trained as a single stem up a cane or string, but these try to grow into more bushy plants and so produce sideshoots. These need nipping out, unless you want to grow lots of leaf at the expense of fruit. Look in the joints between leaves and the main stem and cut, or nip out, any small shoots as soon as they appear there. You should also cut out larger sideshoots, if they have grown unnoticed, but you will make a bigger wound. This may allow disease into the plant. Just remember to check every couple of days (and check round the base of the stem as well). Remove sideshoots while small, for the best plant health.
The first flowers will appear on cherry varieties. To get the best set of early fruit, spray the flowers with a fine mist of water. Subsequent trusses don’t seem to need this extra aid, but if you want the earliest fruit it is worth lightly dampening the pollen. In a hot month, the first green fruit should be swelling by the end of May.
To keep your tomatoes free of whitefly, plant a row of French marigolds close by.

Sweetcorn

Seed sown last month should have produced strong young plants which are ready to put out in the border soil. Don’t leave plants to grow on in pots. Sweetcorn has long roots and these need to go down into the soil in search of nutrients, not round and round the sides of a pot. Plant out 45cm (18in) apart in blocks. This will aid pollination and ensure full cobs. Sweetcorn is greedy – fill planting holes with compost or manure and apply a scattering of dried seaweed on the surface of the soil if you have it. A second sowing can be made now to give a later crop.

 

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