Should You Thin Your Tomato Fruits?

17 July 2009, written by Barbara Pleasant

As gardeners watch their tomato plants load up with green fruits, the question of whether or not to thin them can keep you up at night. To make things worse, thinning methods that work in one climate can prove disastrous in another – so much so that debates between American and European gardeners on the subject have led to nasty conflicts. In order to set the record straight – and help you grow your best crop of tomatoes – here's a global report on thinning trusses of tomato fruits.

It Starts With the Sun

Tomatoes are tropical plants that run on sun, so an abundance of sunshine helps them to grow faster. Where you are on the globe, as well as your area's cloud cover patterns, determine the intensity of sunlight that reaches your tomatoes. Gardeners in the UK and northern Europe (north of the 50th parallel) are just plain out of luck here, because your July solar intensity (insolation) rating is a measly 4.7, compared to 6.0 in St. Louis, Missouri (located at the 38th parallel, close to the center of the US). The farther you live from the equator, the more your plants will be affected by limited sunlight intensity.

The distance from the sun also affects daily temperatures, and low temperatures slow the growth of warm-natured tomato plants. Again looking at July, the average high/low temperatures in Jeremy's garden in Liverpool are 68°/55°F (20°/13°C), while those in St. Louis are 91°/71°F (33°/22°C). Taken together, reduced light intensity and cool temperatures lead to 4 to 5-week maturation lag for tomatoes grown in high-latitude gardens. This difference is reflected in the days-to-maturity ratings given by European vs. American seed companies. For example, 'Sun Gold' cherry tomato needs 95 days to mature in Jeremy's garden, but only 60 days when grown in most parts of the US. Similarly, 'Moneymaker' is rated at 95 days in the UK, compared to 75 days in the US.

Cordons or Cages

With this much understood, it makes sense that high-latitude gardeners should use different methods than gardeners who live in much warmer, sunnier climates.

On the cool-climate side, most gardeners favour handling tomatoes as cordons. European gardeners often substitute "cordon" for "indeterminate" when discussing tomato growth habits, but cordon pruning (to a single trunk) actually describes a method developed by grape growers, and later applied to greenhouse-grown tomatoes.

Here's how it's done with garden tomatoes. Using early, indeterminate (tall, long-bearing) varieties, cordoned tomatoes are pinched and pruned to one or two leaders, and tied to a stake. The plants are allowed to produce up to 7 or 8 flower clusters, which become trusses of tomatoes. Once the green tomatoes have formed, future blossom clusters and suckers are clipped off to help the plants concentrate on ripening their crop. In addition, crowded trusses are thinned so that the ripening fruits won't crowd each other so much that they lose their shapely curves.

Many US gardeners use the cordon method when growing tomatoes in containers or partial shade. However, research studies from Mexico, China and the US have shown that tomatoes in warmer climates produce best when they are allowed to produce a thick foliage cover, as they do when grown in wire cages. Contrary to what many gardeners believe, thinning of tomato fruits in warm summer areas has no effect on the flavour or nutritional value of the remaining fruits, and only a slight influence on fruit size. In fact, fruit thinning may increase problems with fruit cracking, especially if you thin fruits just before a heavy rain. Commercial growers often thin to enhance uniform ripening, but home gardeners have little need for a crop that ripens all at once.

Last-Minute Manoeuvers

Regardless of where you live, the arrival of autumn frost creates an excellent reason to thin tomato fruits. In late summer, tomatoes need 8 to 9 weeks to go from blossom to ripe fruit. If you count back 10 weeks from your first frost date, you know when your tomatoes are starting to waste energy by producing doomed flowers and fruits. My first frost comes in early October, so I start clipping off tomato blooms in mid-August. Tossing beautiful blossom clusters into the compost heap is a hard thing to do, but it helps me understand the resolute, pinch-or-perish attitude high-latitude gardeners must maintain with their tomatoes all season long.

- Barbara Pleasant