JoAnne Skelly: Why are there no tomatoes

Someone asked me this week why there were no tomatoes developing on his plants. The plants look healthy, have lots of leaves, but few fruit are developing.

Poor fruit set on tomatoes, peppers and eggplants, which are in the same family as tomatoes, is influenced by various factors including temperature, relative humidity and soil moisture.

Maximum growth is achieved when temperatures range between 70 and 85 degrees during the day and 60 and 68 degrees at night. Temperatures above or below these optimums curtail growth abort blossoms and impair fruit set.

During the past weeks, we have been experiencing daytime temperatures in the mid- to upper-90s, too hot for happy tomatoes. Thankfully, the nighttime temperatures have been great for tomatoes, staying in the 60s.

When temperatures rise above 90 degrees, many blossoms dry up and fall off without being pollinated. Those flowers that remain on the plant may have pollen (male) and stigmatic surfaces (female) that dry out, causing poor fruit set.

So, the main factor in the current poor fruit set is likely the high temperatures over too many days.

In addition, the strong dry winds have been drying out the flower parts or actually knocking flowers off. Gentle breezes, on the other hand, would actually help in the pollinating process.

Considering that tomatoes thrive when the relative humidity is between 40 percent and 70 percent, we are lucky we ever get tomatoes to set in Nevada, since the relative humidity here is often below 20 percent during the day.

Dry or wet soil can be additional reasons for poor fruit set. The blossoms dry up and fall off when the plants don’t receive enough water. Too much water rots roots preventing the plant from absorbing the water it needs. Soil should be evenly moist, but not soggy, to a depth of 12 inches.

Another reason tomato plants may struggle is the amount of light they receive. Although plants can survive and produce some fruit with only six hours of sunlight per day, yield is higher with eight or more hours.

Too much nitrogen fertilizer or manure promotes leaf growth at the expense of blossoms and fruit. Too little nitrogen also reduces fruit set. Fertilize tomatoes on planting, then after the fruit sets and is one-third of its full size, but still green. Fertilize again two weeks after harvesting the first fruit, and then a month later if we have a long season.

JoAnne Skelly is Associate Professor & Extension Educator, Emerita at University of Nevada Cooperative Extension. She can be reached at skellyj@unce.unr.edu.