What to sow, plant and harvest in your polytunnel or greenhouse in August

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Posted Jul 31 2017 by John Harrison of Allotment & Gardens

In August there is even more bounty than in July, whether you’re growing in a polytunnel or a greenhouse. You may have produced enough courgettes and cucumbers for the whole neighbourhood. The tomatoes are made into soups and frozen and you reap the rewards for all your efforts.

Be aware though, in August there is often an influx of pests and diseases and you are often too busy harvesting your crops and somewhat neglect your plants. It is absolutely important to keep a close eye on your plants during August.


Direct sowing into beds

It is very unlikely that you will find any empty space in your tunnel or greenhouse for direct sowing.

Planting into beds

You can plant all the crops you have sown a month earlier in modular trays into your tunnel or greenhouse:

Calabrese, Chinese cabbage, claytonia, coriander, dill and chervil, Florence fennel, lettuce, oriental brassica salads, pak choi, parsley, scallions.


Sowing into modules / pots (18–20°C)

  • Calabrese (Green Magic F1) – 1 seed per cell
  • Chard (any type) – 1 seed per cell
  • Chinese cabbage (Yuki F1) – 1 seed per cell
  • Claytonia (or Winter Purslane) – 5 seeds per cell
  • Chervil, Coriander, Dill – 5 seeds per cell each
  • Florence fennel (Rondo F1) – 1 seed per cell
  • Lettuce (various types) – 1 to 3 seeds per cell
  • Oriental brassica salads (all types) – 5 seeds per cell
  • Pak Choi (various) – 1 seed per cell
  • Parsley (curly and flat leaf) – 4 seeds per cell
  • Scallions (Ishikura Bunching) – 10 seeds per cell
  • Spinach (annual) – 4 seeds per cell
  • Spinach (perpetual) – 1 seed per cell

Pak choi


In August you are bound to have plenty of excess produce. You have the choice to preserve, to give away or to compost, so be prepared for the glut and warn your friends and neighbours about those courgettes or worse even marrows.

Harvest your courgettes and cucumbers at least twice per week and your tomatoes, peppers and aubergines at least once per week.

You can harvest:

Aubergines, basil, coriander, Chinese cabbage, chillies, courgette, cucumber, dill, Florence fennel, French beans, lettuce, melons, pak choi, peppers, salads, scallions, spinach and tomatoes.

Aubergines growing in a greenhouse in August


General greenhouse / polytunnel maintenance

  • Water more frequently and more heavily now, about two to three times per week, especially during hot spells. Avoid watering in the evening as this will encourage the spread of fungal diseases.
  • Ventilate as much as possible. The tunnel and greenhouse doors should be left open until late in the evening or even throughout the night.
  • Weekly maintenance for summer crops: side-shooting and training tomatoes, cucumbers and melons. You should also remove the lower discoloured or diseased leaves.
  • During August it is most important to check your plants regularly and remove any dead or diseased plants or plant parts, especially grey mould (botrytis) on tomatoes and many other plants as well as keeping a check on the greenhouse whitefly.
  • Harvest regularly when the crops are ready (not as and when you’d like them).
  • Keep a watch out for pests, especially greenhouse whitefly, tomato moth caterpillars, aphids, red spider mite, slugs and snails.
  • Spray aphid susceptible plants with a garlic spray every seven days.


You can get John’s book Vegetable Growing Month-by-Month here.