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Pests of tomatoes

Pests

Pests aren’t usually too much of a problem on outdoor crops, but in greenhouses commons pests include aphids, glasshouse whitefly and red spider mite (properly known as two-spotted spider mite). Commercial growers use biological control agents to manage these pests and these are available on our website: Natural pest control. Soft soap is useful for reducing the numbers of pests – use distilled, rain or soft tap water to dilute it.

There is a list of insecticidal soaps and oils here : Pesticides for Home Gardeners RHS Advisory Service February 2015.

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Pollination and setting of tomatoes

In commercial greenhouses growers use bumble bees to pollinate their crops. If your plants are outdoors, there should be enough wind and insect activity for pollination. In the greenhouse, it is a good idea to gently shake the strings or other supports around mid day, or tap the trusses which have flowers open. If the weather is very hot and dry, it may help to water the paths in order to increase the humidity and cool the greenhouse.

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Watering and feeding tomatoes

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.

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Growing tomatoes to planting stage

Growing to planting stage
Tomato plants need as much light as possible, so keep them in a conservatory or on a sunny windowsill (but don’t leave them behind the curtains). On warm, sunny days you can put them outside, but don’t forget to bring them inside at night. Ideal temperatures at this stage are 15°C at night and 20°C minimum during the day.

Growing to planting stage

Tomato plants need as much light as possible, so keep them in a conservatory or on a sunny windowsill (but don’t leave them behind the curtains). On warm, sunny days you can put them outside, but don’t forget to bring them inside at night. Ideal temperatures at this stage are 15°C at night and 20°C minimum during the day.

Your plants may need to stay in their pots for several weeks until the weather is suitable for planting. Space them so the leaves don’t overlap. They may need liquid feeding, especially if you have used seed compost to pot them up (it is usually lower in nutrients than potting compost). Use a proprietary high potash tomato feed made up according to the label recommendations. Typically they will need feeding from about 4th true leaf stage, but if the bottom leaves start to turn yellow or purple, this can be a sign that they are running out of nutrients (purplish tints can also be due to low temperature).

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Pricking-out tomatoes

Although you can successfully plant the seedlings directly into the soil, we recommend that you pot them on first.

You will need one pot for every seedling – at this stage allow about 10% extra. Pots with a diameter of 13cm (volume 1 litre) are a good size. You can use the same compost that you used for sowing, but you may need to liquid feed the plants if you are growing them for several weeks in the pots.

Fill the pots to the brim as the compost will settle once it is watered. Make a hole deep enough to take the roots without curling. Plant them so that the cotyledons are just above the surface of the compost. Do not firm them in – water them in gently. Check them every day and give them a light watering as required. Make sure the leaves are dry before evening, as the seedlings are more likely to ‘damp off’ (rot) if they go to bed with wet leaves.

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Greenhouse or outdoors for growing tomatoes?

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.