Caring for potted tomatoes in hot weather

Q. In the past two summers my potted greenhouse tomatoes grew and flowered well but set very little fruit. The greenhouse becomes over-heated in hot weather despite shade cloth, open vents and misting several times a day. I’ve also never been sure how much to water.

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A. This issue has become a major problem in the hot summers of recent years. Temperatures at and over 30 C will sterilize tomato pollen and put a stop to fruit set.

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Adding a fan to your shading, misting and ventilation measures would help, but the only sure way to ensure fruit set during very hot weather is to move the pots to a cooler site.

How often and how much to water depends on variables such as the size of the pot, the water retention quality of the potting mix, and the size of the plant. It is far easier to keep the soil adequately and evenly moist in large rather than in small pots.

I plant very compact patio tomatoes in pots around 25 cm wide. Larger tomato plants need pots in the 30 to 45 cm range. I set my patio tomato pots in trays, and water into the trays after I’ve watered the soil — every day in hot weather. Lifting the pots, where possible, to sense the weight, or poking a finger into the soil, are low-tech guides to moisture needs.

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Some purchased potting mixes are very light-weight and have less than ideal moisture retention capacity. Adding some hefty-textured, all-purpose potting soil enhances the “staying power” of such blends.

These days, product availability fluctuates, but my habit has been to use as the base for my potting mixes Pro-Mix BX (for “basic”) and then, for increased heft, I add about one-third as much (as the Pro-Mix) of a sterilized, all-purpose potting soil of some weight and substance.

I usually mix in some coir (coconut fibre) as well, for its buffering ability against temperature extremes and added moisture retention.

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