Tomato trouble? 5 common mistakes that might be hurting your harvest

The mistake: feeding too often or too early

What it means for your tomatoes: tomatoes need to develop a good root system before they can absorb the nutrients from tomato feed. Feeding too early can cause too much leaf growth and also delay flowering as the plant will put all its energy into producing foliage. It can also reduce the growth of roots. Feeding too often can lead to burnt roots and too much leaf growth at the expense of fruit production.

What the expert says: "Once a week is enough for tomatoes grown in pots or growing bags, rising to twice weekly in late summer. Those in outdoor border or greenhouse soil can get by with once a fortnight if the soil was generously prepared with compost before planting."

How to get it right: Start feeding tomatoes once the first flowers begin to form fruit. Adjust how often you feed tomatoes depending on where they are growing and how far it is through the growing season.

Giving the wrong feed

The mistake: giving your tomatoes plant foot that doesn't suit their needs

What it means for your tomatoes: tomatoes need high levels of potassium for their developing fruits. A lack of the right nutrients can lead to ripening problems such as greenback (hard patches on the fruit), pale patches, or small fruits. Magnesium deficiency can cause discolouration on the leaves and mottling.

What the expert says: "Tomatoes have a very great need for potassium when the fruits form and ripen so high potassium tomato feeds are ideal for flavoursome crops of quick ripening toms."

How to get it right: buy a plant food that is specifically for tomatoes as this will contain the right balance of nutrients

Letting sideshoots take over

The mistake: letting sideshoots develop instead of removing them regularly.

What it means for your tomatoes: this can result in a plant with a mass of stems and fewer fruits as the plant puts more energy into producing foliage and stems than it does to growing fruits. On cordon tomatoes the plant forms one vertical stem whereas bush tomatoes are compact and produce fruit and flowers on their sideshoots.

What the expert says: "Some tomatoes, 'bush tomatoes' are bushy and each shoot ends in a flower so don't side shoot these. Others grow madly with flowers forming on the sides of the proliferating stems as they grow. This creates a terrible muddle and to avoid this the main stem is tied to a 1.5km stake or string and the sideshoots are removed as the plant grows until it reaches the top of its support."

How to get it right: check tomatoes plants regularly for any shoots developing in the joint between the leaf and the stem and remove them

Irregular watering

The mistake: watering too much or too little, or uneven watering

What it means for your tomatoes: erratic watering can lead to problems with tomatoes including split fruit and blossom end rot. Split fruit can also go mouldy. Too little water can result in fewer fruits, nutrient deficiencies and small fruits, but too much water can dilute the flavour of tomatoes. Tomatoes need regular and consistent watering. A consistent supply of water helps the plant to take up nutrients, which in turn results in a good harvest with flavoursome tomatoes.

What the expert says: "Ideally grow tomatoes in border soil so they can explore the soil for moisture as they need it. However, practically, it is often necessary to use pots or growing bags. Pots, filled with growing bag compost are best especially if you can use bigger pots, 45cm diameter say, when daily watering often suffices. Growing bags are tricky as they can need watering three times a day. Extensions to hold more compost are available which help somewhat or an automatic watering system can be used. Kits are marketed for this purpose."

How to get it right: aim to keep the compost moist by watering regularly but be careful not to overwater. The best time to water tomatoes is in the early morning or evening to avoid the water evaporating in the heat. Look out for signs that tomatoes need more water, such as curling leaves, the plant wilting or dry soil around the plant. Signs of overwatering include yellowing leaves, wilting, split fruit as well as bumpy or blistered leaves.

Not watching out for pests

The mistake: not checking regularly for pests that might be affecting tomato plants

What it means for your tomatoes: atomato moth can be a problem when growing tomatoes under cover, as can sap sucking insects such as whitefly, while aphids can affect tomatoes outdoors. However small infestations of aphids and whitefly will not usually affect crops in low numbers. Birds will often feed on aphids. Large infestations of these insects can weaken plants, causing curling leaves and stunted growth, which can then affect yields.

What the expert says: "Tomatoes have few unwelcome visitors – the foliage and stems are so hairy that most creatures are not keen on them. Tomato moth can bore into fruits in greenhouses – its inconspicuous nocturnal caterpillars can be picked off at night by torchlight. Potato blight disease can be a menace outdoors in wet seasons by late summer – choose tolerant varieties which can usefully prolong the cropping period."

How to get it right: check tomatoes regularly when watering or feeding to keep on top of pests. Small infestations are easier to deal with. Squash aphids by hand and try planting strong smelling companion plants such as French marigolds to deter pests like whitefly. There is also a biological control for whitefly, which can be affective for large infestations.

Next steps: how to get green tomatoes to ripen

• Remove the top of your tomato plant once it has three or four trusses of fruit
• Don't attempt to ripen tomatoes unless they have already started ripening naturally
• Cut off some leaves to allow more sunlight to reach any tomatoes left on the plant
• Put tomatoes in a fruit bowl or a drawer with bananas or avocados as these release ethylene, which will help them ripen

What the expert says: "Gather tomatoes when nights get chilly and bring into the warmth where they can ripen. Ripening is promoted by a gaseous plant hormone called ethylene. This hormone is emitted by ripening fruits and although not essential tomato maturity can sometimes be hastened by placing the more mature green tomato fruits in a paper bag with a ripening tomato or banana. Young, very green fruits won't ripen no matter what – chutney beckons for those."

More on growing tomatoes:

• How to grow your best-ever crop of tomatoes
• Seven things you need to know about tomatoes