This can happen for a number of reasons, but a common cause is temperature extremes.
Early in the season, when night temperatures drop to 55-60 degrees, or are excessively hot (70-75 degrees), the blossoms will abort.
Hot day temperatures are also a problem. Depending on the variety, some may not set fruit once we get over 85 degrees, while it might take 90 or slightly above for others. I can promise you that they will all abort when we have 100-degree temperatures, as occurred earlier in the week.
Some tomatoes are "heat set" varieties that have been bred for high day and night temperatures common in the summer.
An important factor involved with temperature is time of exposure. The longer the plants are exposed to high temperatures, the more severe the effect on flowering. Short exposures such as a week or less should not cause much of a problem.
Tomatoes are pollinated by movement, not by bees, so when the wind blows, tomatoes pollinate themselves. A lot of consecutive still days can cause blossoms to abort, but we've been having plenty of wind, so this should not be the problem .
Another possible cause is too much fertilizer, particularly too much nitrogen (first number in a fertilizer analysis), especially when it is applied at or closely to flowering. Excessive nitrogen in the soil promotes leaf growth at the expense of blossom and fruit formation. The best granular fertilizer is 5-10-15, while 10-10-10 is acceptable.
Don't use a liquid fertilizer designed for evergreen houseplants, such as 20-20-20. There are liquid fertilizers with lower nitrogen numbers, and higher phosphorus and potassium that are formulated for vegetables.
Blossoms also dry up and fall off when the plants don't receive enough water. This is especially critical on those days when the temperature soars into the upper 90s or 100s.
Excessive shading does not cause blossoms to abort, the plants just does not produce as many. Tomatoes should probably get at least eight hours of full sun a day. Fewer blossoms are produced when the plants receive less than six hours of sun a day.
What should you do? Try to keep the plants as healthy as possible. Keep them watered, maintain fertilizer levels and control any pest s, as any additional stress will make the condition worse. The plants will produce flowers and set fruit when temperatures are more favorable.
Garden centers sell tomato and vegetable blossom set spray, which is a natural plant hormone that is supposed to make blossoms set fruit earlier and more consistently, in spite of the weather. You might try using this, but based on the feedback I have received over the years, it does not give consistent results.
Blossom End Rot
Blossom end rot causes the fruit to have a dark, sunken area on the blossom (bottom) end of the tomato. Technically, this disorder is associated with a low concentration of calcium in the fruit. It tends to occur when there is drought stress followed by excessive soil moisture, a s these fluctuations reduce uptake and movement of available calcium. It's common when you are growing tomatoes in pots and let the soil dry out too much between watering.
To manage blossom end rot:
-Maintain the soil pH between 6.0 and 6.5 and supply adequate levels of calcium through applications of dolomitic lime or gypsum.
-Avoid drought stress and extreme moisture fluctuations by using mulch and deep, timely irrigation twice or three times a week. Depending on the temperatures, some tomatoes may need watering every other day or every day when we hit triple digits. Those growing in pots may need watering twice a day.
-Avoid overfertilizing plants with high ammonia nitrogen fertilizers. Excessive nitrogen can depress the uptake of calcium.
-Products such as Stop Rot are only short-term fixes and often work poorly because of poor absorption.
Reach Sid Mullis, the director of the University of Georgia Extension Service office for Richmond County, at (706) 821-2349 or firstname.lastname@example.org.