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Training Tomatoes: How to Run a Sugar Factory

Training Tomatoes: How to Run a Sugar Factory

By Rob Sproule

Before you cut
Tomato Behaviour
Rules of Thumb

“Knowledge is knowing a tomato is a fruit; wisdom is not putting it in a fruit salad.”
– Miles Kingston

Before you cut:

Tomatoes are far and away the most popular backyard edible, with almost every gardener enjoying at least one of them. But they’re quirky plants, too. We think of them as bushes, but most are vines, pure and simple. And we need to treat them like vines. Before diving in like Edward Scissorhands, check the tag your tomato came with.

If it says “Indeterminate”, then commence your slashing. If it says “Determinate,” back off. Determinate tomatoes grow like a bush. They get a certain height and stop. They’re well behaved, perfect for patios, and produce a fraction of what their belligerent, vining cousins produce. If you’re planting en masse, separate the two types upon planting so you know which ones to train. We prune to increase yield, but pruning Determinate tomatoes will do the opposite.

Tomato Behaviour:

A little chemistry, first. Tomatoes are sugar factories. Their leaves exist to convert light into sugar, which the plant then uses to make its delicious fruit (yes, it’s a fruit). Your job as trainer is to make the sugar factory as efficient as it can be. Do that, and ripe red tomatoes will roll off the assembly line as fast as you can eat them.

If you don’t prune your tomato, it will produce more leaves, send out more shoots and growing tips, and basically try to catch a ray of sunlight wherever it can. The leaves will get so dense that each leaf will catch only a fraction of light, and all that foliage will drain precious resources. By midsummer, it will become a tangle of green with not a juicy tomato in sight.

Learn more about training your tomatoes, with Alberta’s Best Gardening Blog

Rules of Thumb:

The few stems you have, the more efficient your sugar factory will be. With one stem, all the sugar the leaves produce will go into fruit production and you’ll get the biggest tomatoes. You never want more than 4 stems on an indeterminate tomato. Growing a new shoot diverts energy from where you want it spent: fruit production. More leaves also means that each leaf is getting less sunlight. Less sunlight means that each leaf is producing less sugar. Leaves take a lot of energy to grow, so if they don’t produce as much your “ROI” is lower per leaf (I told you this was about efficiency).

You’ll need to support your key growing vines, especially when they’re heavily laden with tomatoes. Secure the vines to your trellis or tomato cage with soft twine or tomato fasteners. Never use wire, elastics, or anything that would cut into the delicate stem and invite fungus in. Ideally, you’ll have one or two well supported vines with all leaves bathed in sunlight. This will flood the plant with abundant sugars, all of which it will invest into juicy goodness.


You want to avoid any leaves (or fruit, of course), sitting in the dirt. That’s an invitation for deadly fungus to spread upwards. Lower leaves tend to be bigger, thicker, and less efficient. I prune off the bottom 20% of leaves as a rule. If you don’t, they’ll become a waste of resources, eventually turning yellow as the rest of the plant, seeing that they’re not producing sufficient sugars, chokes them out.

Most tomato fungal problems are caused from wet soil splashing onto lower leaves. If it’s dense down below, the issue gets worse quickly. A supported tomato will have fewer issues thanks to more air movement around the leaves and being elevated. Branches tend to grow at a 90 degree angle from the stem. You’ll notice side shoots peeking their heads out between them at about a 45 degree angle. Nip these guys off as soon as you see them. Suckers often appear near ground level and grow straight up. They’re a drag on your factory’s resources, nip them off asap.


Read through our Growing Guides for tips to enrich your garden! 

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