If you are growing in a greenhouse or polytunnel make sure there is a good air flow to avoid temperatures above 26°C, if possible. You want to avoid condensation forming on the plants as this encourages disease.
Getting the watering right in a grow bag or pot is always more difficult than if you grow tomatoes in soil. If you have a small greenhouse and always grow your tomatoes in the same place, then there can be a build up of soil-borne diseases such as Fusarium (wilt; crown and root rot), Verticillium wilt and Corky root rot. You can get round this either by not growing in the soil or by using grafted plants: see To graft or not graft for tomato growing?
Soil preparation for the tomatoes
Tomatoes prefer a well-drained soil or growing medium which encourages a deep and healthy root system. Organic matter is essential to develop and maintain good soil structure – well-made garden compost is ideal.
Tomatoes prefer a well-drained soil or growing medium which encourages a deep and healthy root system. Organic matter is essential to develop and maintain good soil structure – well-made garden compost is ideal. The best way to find out the nutrient status of your soil is to have it analysed – The RHS offers a soil analysis service. Tomatoes prefer a soil pH in the range 6.0 – 6.5 i.e. slightly acid, but this is a bit too low for some crops such as lettuce and brassicas, so you might need to aim at between 6.5 and 7.0. However don’t apply lime immediately before planting tomatoes unless the pH is below 6.0.
You should aim to apply all of the Phosphorus and Magnesium needed for the whole season, part of the Potassium (also known as potash) but little or no Nitrogen, or else you will get lots of lush growth and your flowers won’t ‘set’ fruit. Don’t forget that your garden compost should contain a lot of nutrients. In the absence of a soil analysis, apply a high-potash fertilizer e.g. Chase Animal-Free Tomato Fertiliser according to the recommendations on the packet; fork in the fertilizer or compost to a depth of about 20cm.
If you are using grow bags make sure they are placed on a firm surface, slightly sloping away from your path. If you’re using pots or other containers, the bigger the better, not less than 12 litres per plant; fill them with a good quality peat-free growing medium recommended for containers such as Fertile Fibre Multipurpose Compost or Melcourt Sylvagrow (peat-free, but not organic), both Gardening Which Best Buys.
Water the plants in well to settle the soil around the roots (this is known as ‘ball watering’). Thereafter you need to adjust the watering according to the size of the plants and the weather. Giving plants a thorough water rather frequent light sprinklings will encourage deep rooting in the early stages, but once the fruit start to swell you want to maintain an even moisture in the soil as erratic watering can cause blossom-end rot and fruit splitting. However, you want to keep the plants on the dry side as this gives smaller, more highly-flavoured fruit. You can feed every watering with a dilute high potash feed, or once a week with a more concentrated dose – follow the instructions on the packet e.g. Neudorff 1L Organic Tomato Feed or make your own liquid feed from comfrey or nettles: instructions here.
Give your greenhouse a good wash down before planting to maximise light transmission and get rid of disease spores.
Late (potato) blight can damage or even kill outdoor tomatoes and spoil the fruits before you can harvest them, but it is less of a problem if you grow your plants in a greenhouse or polytunnel. If you have to grow outside, choose the sunniest spot you can find (e.g. in front of a south-facing wall). There are also a few varieties which are more blight-tolerant; we have selected six of the best for our Blight-tolerant Outdoor Tomato Collection.
Tomatoes will not stand a frost – keep your seedlings indoors or in a heated greenhouse until the risk of frost has passed – sometime in May for most areas, but check your local weather forecast.