My tomato plants are growing in bags. They are very tall but have started to go yellow and die.

Can you help?

There could be a few reasons your tomato plant isn't thriving and so it will be a process of elimination.

If it is the lower leaves that are yellowing and dying, this is normal for tomatoes at this time of year, if it is the new growth then the plant likely needs feeding and consistent watering.

Is the grow bag or pot they are growing in large enough? The minimum size for growing tomatoes in pots should be a 50-60cm pot. This means there will be enough potting mix to continue to feed the plant and to hold enough moisture after watering. 

Was the tomato planted into a quality, free-draining potting mix? Compost or garden soil is not a suitable potting mix for growing tomatoes in pots as it compacts as it breaks down. Potting mix has the right blend of nutrients and allows water to drain freely.

Potting mix contains a controlled release fertiliser as well as trace elements that release nutrients as plants grow. It's always a good idea to give tomatoes an additional feed with a liquid fertiliser as they do a lot of growing in a short space of time and quickly use up the fertiliser. Tui have a couple of options for supplementary feeding, Tui Seaweed & Fish Liquid Fertiliser or Tui Superfood Tomato Liquid SuperFood. These are mixed with water and applied to the potting mix weekly.

Over watering or under watering - is the plant being consistently watered? Plants in pots may need watering daily, or every second day, depending upon the weather. Give plants approximately 5 litres of water per water, until the water starts running out of the bottom of the pot/bag.

When watering avoid wetting the foliage as this can lead to fungal disease problems such as blight or powdery mildew. The leaves turn brown and die, it usually affects the lower leaves. Remove these leaves back to the main stem of the plant. Improve air circulation around the plant to get better air flow, this reduces the humidity and reduces the chance of diseases such as blight affecting the plant.

Is the plant getting enough sunlight? Tomatoes require approximately 6 hours sunshine a day, tall leggy growth is an indication that it may not be getting enough sun, if you can, move the pot to a sunnier position. Make sure it is sheltered and protected from the wind.

We'd also suggest staking the plant to support the new growth.

I hope these suggestions are helpful.