Blossom drop is a common tomato growing problem that can be extremely frustrating to the home gardener. What happens is that otherwise healthy looking tomato plants set flower blossoms, then they just dry up and fall off the plant before a fruit is formed. There can be many causes of blossom drop, and the most common is temperature.
What Causes Blossom Drop in Tomatoes?
Many of the problems that cause blossom drop can be difficult to control because they are related to temperature and plant stress:
- Temperature too high or too low
- Lack of pollination
- Nitrogen—too much or too little
- Humidity too high or too low
- Lack of water
- Stress from insect damage or disease
- Excessively heavy fruit set
The most common cause of tomato blossom drop is temperature. Tomato plants are stressed if the high daytime temperature is above 85 F, the high nighttime temperature is above 70 F, or if the low nighttime temperature is below 55 F.
Tomatoes grow best if daytime temperatures range between 70 and 85 F. While the plants can tolerate more extreme temperatures for short periods, several days or nights with temps outside the ideal range will cause the plant to abort fruit set and focus on survival. According to the University of Nevada Cooperative Extension, temperatures over 104 F sustained for four hours can cause tomato flowers to abort.
Controlling Tomato Blossom Drop
Nothing will guarantee fruit set. And because factors like temperature and humidity are out of the gardener's control, sometimes you just have to be patient and wait for conditions to correct themselves. If the weather seems fine and other gardeners in your area are not having fruit set problems, then you should consider the cultural causes of tomato blossom drop. Choosing a suitable variety and keeping your plants healthy will give you an edge:
- Grow varieties suited to your climate. Gardeners in cooler climates should not rush to get their tomatoes planted in the spring. Wait until nighttime temperatures are reliably above 55 F, or protect them with a cover at night. You won't gain any advantage by setting them out too early. Choose early-maturing tomato varieties for spring growing in cooler climates; examples include Early Girl, Legend, Matina, Oregon Spring, Polar Baby, and Silvery Fir Tree.
Conversely, select a heat-tolerant ("heat set") tomato variety for areas with long periods of hot or humid weather. High nighttime temps are even worse than high daytime temperatures because the tomato plant never gets to rest. Heat-tolerant varieties include Florasette, Heat Wave, Solar Set, Sunchaser, Sunmaster, Sunpride, and Surfire.
- Ensure pollination.
Tomatoes need some help to pollinate. Either insects, wind, or hand-shaking of the flowers is necessary to carry the pollen from the anthers to the stigma. During weather extremes, there are often no insect pollinators in the garden. One way to attract more bees is to plant nectar-rich flowers in your vegetable garden.
- Go easy on the fertilizer.
Don't automatically feed your tomato plants every week. Make sure your soil is healthy, with adequate organic matter. Apply a balanced fertilizer at planting and again when the tomatoes start forming. Too much nitrogen encourages the plant to grow more foliage, not more fruit.
- Work around the humidity.
The ideal humidity range is between 40 and 70 percent. If humidity is either too high or too low it interferes with the release of pollen and with pollen's ability to stick to the stigma so that pollination does not occur.
If humidity is too low, hose the foliage during the day. This will both cool the plant and raise the humidity. However, this is not recommended in areas with high humidity or when fungus diseases are present. Gardeners in high-humidity areas should look for tomato varieties that aren't bothered by humidity, such as Eva Purple Ball, Flora-Dade, Grosse Lisse, Jubilee, Moneymaker, Sun Gold, Taxi, and Yellow Pear.
- Water deeply, once a week, during dry weather.
- Tomatoes have very deep roots, sometimes going down 5 feet into the soil. Shallow watering will stress and weaken the plants.
- Keep your tomato plants healthy.
Use good cultural practices and treat for disease as soon as symptoms appear.
- Sometimes the problem is just too much of a good thing.
When a tomato plant has too many blossoms, the resulting fruits are all competing for the limited food supplied by the plant. Only the strong will survive. The plant will automatically abort some flowers (much like June Drop of tree fruits). Once the initial crop is harvested, the problem should subside.