The Truth About Pruning Vegetable Plants

29 June 2023, written by Barbara Pleasant

Most vegetables are fast-growing annuals with one goal: to produce a robust crop of seeds. It happens in stages. Leaf by new leaf, juvenile plants gain energy for growth through photosynthesis until there are enough leaves to support reproduction. As the plants mature, the canopy of leaves shades flowers and fruits from intense sun, helps shed excess rain, and limits surface evaporation in times of drought.

It's a beautiful plan, with little or no need for the gardener to intervene with pruning shears. Most of the time. There are situations where pruning can be beneficial to plant health by improving light exposure or thwarting pests and diseases. And should you wish to grow a giant pumpkin or tomato, you will need to use special pruning techniques to increase fruit size.

“Aphids” — The best way to deal with tight aphid colonies on tender buds and growing tips is to snip them off

Beware of over-zealous pruning advice. In recent years much has been learned on how greenhouse cucumbers and tomatoes can be pruned to maximise productivity, the key word here being 'greenhouse'. These are special varieties grown in robotically managed, low-stress commercial environments that are far different from the wild world of the garden. Garden-grown plants need a wider margin of error, and the capacity to bounce back from storms or other traumas, so they benefit from keeping most of their stems and leaves. When pruning vegetable plants, you should have a sound reason for making each cut.

“Alt — Pruning low foliage from tomato plants helps reduce the spread of early blight and other diseases

Pruning to Manage Pests and Diseases

It’s a bad aphid year in the US, with gardeners reaching for spray after spray to control them. While I’m all for aphid management, a first line of defence is to prune off infested plant parts. A bud or leaf that has been badly damaged by aphids, spider mites or other sucking insects will not recover, so it is best removed and dispatched to the compost pile.

In every garden there are diseases that return every year no matter what you do. Growing varieties with genetic resistance means your plants stay healthy for longer, and preventative pruning can further reduce risk by improving air flow and light penetration. In my garden, tomato early blight always causes the lowest leaves on my plants to turn brown and wither, so I go ahead and prune off those leaves once the plants show vigorous growth. Similarly, when older leaves of summer squash start going gray with powdery mildew, it makes me feel better to lop them off and throw them on the compost.

“Powdery-mildew” — Squash leaves infected with powdery mildew will not recover so are best pruned off and composted

Indeed, there is rarely a good reason to keep low leaves that may shelter slugs or serve as incubators for leaf spot diseases. Most vegetable plants naturally shed their oldest leaves, so pruning off those that show signs of decline simply speeds things up a bit. Just don’t get carried away and start lopping off healthy leaves.

Pruning to Manage Size

Growing exuberant vining vegetables is fun, but sometimes pruning is needed to train the stems where you want them to go. Try to train rather than trim when working with young plants, which need every leaf to support strong growth. With older plants such as super-vigorous cherry tomatoes, sometimes a hard pruning in late summer followed by deep watering will push out a flush of new growth. Don’t go overboard when pruning tomatoes, because over 80% of the sugars in the fruits come from photosynthesis done by the leaves. Maintaining a high leaf-to-fruit ratio is a sure route to great tomato flavour.

“Winter-squash” — Winter squash vines can be pruned by about 20 percent if needed to keep them in bounds

If you have a pumpkin or watermelon vine taking over your back yard, you can prune a little, up to about 20 percent of the plants’ total mass. Whether leaves are lost to pests, pruning, or both, most vegetable plants can lose 30 percent of their foliage and still produce well.

Pruning to Increase Fruit Size

Want to grow a giant pumpkin? How about a five-pound tomato? In addition to starting with an appropriate variety and creating a dream planting site, you will need to prune off early and late fruits to force the plants to direct more energy to goliath specimens. For those who enjoy spoiling and pruning vegetable plants, growing giant vegetables may be the perfect hobby.