Fruit Thinning of High Tunnel Tomatoes

Published: March 1, 2011

Fruit thinning is a common practice in tomato production using protected climates in many Asia and Europe. By doing so, the number of desired marketable fruit such as No. 1s will be produced. It also increases light exposure to fruit and decreases fruit disease incidence because of the increased space between fruits. In most cases fruit will change color earlier.

Picture 1: The rule of thumb for a single-leader tomato plant is to keep 2-4 fruit/cluster for varieties of ½-¾ lb/fruit.

Picture 2: The lower one or two fruit can still be removed.

Picture 3: Keeping 4-6 berries/cluster should applicable to most varieties.

Fruit thinning is more applicable to indeterminate varieties and more important for the first couple of clusters. In the early growing stage, berries face more competition with new flower clusters, leaves and branches (suckers). This, in combination with the adverse environment in March and April, discourages fruit set and then the growth of “set” berries, which compete with each other for nutrients. By removing certain number of berries, the growth of remaining berries will be enhanced. While the number of berries to keep depends on varieties and soil fertility, the rule of thumb for a single-leader tomato plant is to keep 2-4 fruit/cluster for varieties of ½-¾ lb/fruit (Picture 1), and 5-6 berries for varieties with single fruit weight of less than ½ lb. For a double-leader plant, the number could be as low as 2 berries/cluster. The time to thin a fruit cluster is after fruit-set and a berry has grown to the size of kidney bean. Flower cluster thinning is not recommended in early spring as fruit abortion could occur. If you haven’t thinned and the tomato fruit has already been developing for 2 to 3 weeks or even longer, it still benefits to do fruit thinning. The lower one or two fruit can still be removed (Picture 2).

Thinning the rest of fruit clusters will depend on the time available to a grower. If time permits, keeping 4-6 berries/cluster should applicable to most varieties (Picture 3). This, implemented with a good fertility management, would allow plants to continuously produce fruit and avoid poor fruiting in mid-clusters (the 6th to 8th). If your tomato plants were grafted onto a vigorous rootstock like "Maxifort", the number of fruit to keep could be more by one or two berries per cluster. Grafted tomatoes usually do not have the poor fruiting part in the 6th to 8th clusters because of the strong root system.

Anchoring the ribs of a high tunnel properly during construction also helps strengthen it. In areas of high snow load or wind, anchoring every second rib in a concrete caisson is advisable. The caisson should be at least 24 inches in depth to prevent "frost heaving". Additionally, equipping the ribs of a high tunnel with cross members to form an A truss, greatly increases its strength. This can be done by securely attaching a metal pipe inside of the high tunnel from one side of the rib to its other, above head height or about eight feet from the ground.