It helps to know a few simple facts. For example, there are two main types of tomatoes: determinate and indeterminate. Bush tomatoes are determinate - they grow, produce fruit then die back - but cordon or indeterminate tomatoes keep on growing until you pinch out the tops or frost kills the plants.
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Thanks to an amazing summer, my Red Alert bush tomatoes produced a large, wonderfully sweet crop until the middle of this month before making space in the polytunnel for winter salading. During more normal summers, I dig up the plants in mid September and am often left with a few green ones that still need to be ripened.
What, though, is the best way to get the most out of cordon tomatoes grown in the greenhouse? Well, it seems fairly obvious, but clean windows are essential, especially now the sun is beginning to weaken. It's been estimated that clean windows allow 48 per cent more light into the greenhouse, encouraging photosynthesis and making the place a bit warmer. So, if you live near a busy traffic route, have a coal fire or a large spider population, get cleaning.
Light quality is critical. It is most effective when passing directly through glass at 90 degrees. It's estimated that, for most of the year, more light enters a greenhouse when the glass is angled. A vertical glass wall lets less light through. This is obviously paramount during winter when sunlight is at a premium. As far back as the 18th century, designers of early greenhouses appreciated this. In 1718, a Dr Boerhaave from Leiden built a greenhouse that had walls angled at 52 degrees, reckoning it allowed the light to penetrate the glass at 90 degrees.
He wasn't far off the mark. Modern research suggests that, for most of the year, the best angle for glass is 60 degrees. In 1948, WJC Lawrence of the John Innes Institute found that these angles on south-facing walls increased greenhouse temperatures by 63 per cent. Greenhouses - and polytunnels - attract more light and warmth when one side faces due south. And Lawrence found these east-west greenhouses with angled walls increased light intensity by 27 per cent and were 63 per cent warmer than vertical ones.
Although you can't change the angle of your greenhouse walls, you can help tomatoes in other ways. They need the sun and a steady flow of warm air for ripening, so it's best to remove leaves below the lowest fully grown truss. Repeat this for each of the higher trusses when you think the tomatoes have stopped growing. This lets the sun in and helps provide good air circulation.
The general advice is to pinch out the growing point of the plants once they've produced four trusses. But when you stop depends on the season, whether the greenhouse is frostproof, and whether you need space for other crops. In summer, tomatoes take eight weeks from flower to harvest. So, once the fourth truss has formed, work out if you can expect more fruit.
You may be able to extend the season into late October or November. In the autumn, growing slows down to 10-12 weeks, so you should nip out the tops now to enjoy late autumn fruit. Sadly, with less sun, these late tomatoes won't be quite as tasty. But, as a matter of principle, I try to leave the last few tomatoes on the vine until Christmas. It's fun to have a tomato salad on Boxing Day, even if you have to imagine that summer flavour.
If frosts threaten or you require the space, you'll have to pick some tomatoes that are still green. All is not lost: though they're a bit sharper, they fry perfectly well. They will also ripen slowly on a window sill, but you can speed up the process with a ripening banana (see panel).