Blossom Drop: Why Heat Stops Fruit Set

Blossom drop is not blossom-end rot

The two get swapped constantly, and the fixes do not transfer. Blossom drop happens before there is any fruit. The flower opens, nothing pollinates it, the small stalk holding it (the pedicel) turns yellow, and the whole flower dries and falls from the plant. Blossom-end rot happens after a fruit has set. Wisconsin Extension describes it beginning as water-soaked spots that resemble small bruises, most often on the bottoms of fruits, which enlarge into a dark brown to black patch that is sunken and leathery.

One is a pollination failure in a flower that never became a fruit. The other is a calcium failure inside a fruit that already exists, and usually a delivery failure rather than a shortage. Wisconsin allows that low soil calcium can be the cause, but says that more often there is plenty of calcium in the soil and it is the uptake and transport to the fruit that is impaired. Alabama Cooperative Extension is blunt about which disorder is which: blossom drop is unrelated to any nutritional disorder or disease or insect damage, and is related to weather conditions, particularly air temperature.

The calcium remedies aimed at blossom-end rot, the crushed eggshells and the garden lime, will not bring back a dropped flower. There was never a fruit there to feed. Epsom salt, which usually gets thrown in alongside them, is not a calcium remedy in the first place: it is magnesium sulfate, and Wisconsin lists high soil magnesium among the things that impair calcium uptake.

The temperatures that stop fruit set

The published numbers do not all agree, and the spread is the useful part. Purdue's Vegetable Crops Hotline, writing for high tunnel growers, calls 60 to 90F by day and 60 to 75F at night the optimum band for fruit set. That is the loosest line anyone here draws. The other services draw it tighter, inside Purdue's optimum.

Alabama Extension: when day temperatures exceed 85F and night temperatures exceed 72F, tomato flowers often abort. University of Missouri: when daytime temperatures exceed 85F or nighttime temperatures exceed 70F, pollination suffers. University of Maryland Extension, writing during a mid-Atlantic heat wave: daytime highs of 90F and above with nighttime lows only getting down to 70F will cause blossom drop and fruit abortion in tomatoes. South Dakota State University puts the pair in the same place, 90F by day with 70F at night.

One caution on counting sources. Maryland says this twice, in a 2015 field bulletin and in a web article updated in 2024, and the same extension vegetable specialist wrote both. The 2015 wording is that those temperatures may cause blossom drop and fruit abortion. The 2024 wording is that they will. That is one source hardening its language, not two independent observations.

Night is the number people skip. Alabama notes that for some varieties, night temperatures alone above 72F can be detrimental to flowering even if day temperatures stay below 85F. Purdue sets its night ceiling higher, at above 75F. A hot afternoon that cools off after dark is a different situation from a merely warm afternoon that never cools.

Duration counts as much as the peak. Alabama's guidance is that the longer plants sit in high air temperatures, the longer the condition lasts and the more serious the effect on flowering, and that short exposures, a week or less, should not cause much of a problem. Maryland puts the same idea in field terms: tomatoes tolerate extremes for short periods, but several days or nights above 86F by day or 70F by night will make the plant abort flowers.

Heat is not the only trigger, either. Alabama and Missouri both note that temperatures below 55F inhibit fruit set at the other end of the range, and Alabama adds that over-applying any fertilizer raises salt levels around the plant, which can cause blossom drop by itself. This page is about the summer failure. A cool coastal garden, and a heavily fed one, can lose flowers without a heat wave.

What the heat actually breaks

The pollen goes wrong. Maryland Extension describes pollen that becomes sticky and nonviable, which prevents pollination from occurring. Missouri uses the same picture, pollen turning tacky and non-viable.

The timing is what confuses people. Purdue reports that the effects of high temperature on fruit set fall primarily on the stage of pollen development, which occurs about nine days before flowers open. So some of the flowers dropping today were compromised during a hot spell that may already have ended. Primarily is not only, though, and Purdue names two more mechanisms that act at or after the flower opens. Heat changes flower structure by causing stigma exertion, pushing the stigma out where pollen cannot land on it. And after pollination, pollen germination can be severely reduced above 100F.

Once a flower does open, the window is short. Maryland puts it at roughly two days from the point the flower becomes viable. Pollination happens inside that window or the flower aborts.

Heat is not the only way pollen fails, either. The same Maryland bulletin notes that relative humidity above 80 percent during pollen shed keeps pollen from releasing properly, which gives poor or incomplete pollination. A muggy stretch can cost fruit without a single record high.

Not the bees, and not the calcium

Two misdiagnoses dominate. The first blames a pollinator shortage. Tomato flowers are mostly self-fertile, and outdoors they pollinate themselves without help. Maryland Extension: ordinarily in tomato fields pollination is achieved just by the action of the wind, with pollen released from the flower and falling onto the stigma. Missouri describes the same transfer, and notes that indoors, where there is no wind, growers substitute for it by mechanically shaking the flowers, or by bringing in bumble bees in larger greenhouse operations.

A flower dropping in a heat wave is not waiting for a bee to arrive. Its own pollen has stopped working, and no visitor can fix that. Planting more bee forage is good for the rest of the garden and will not put tomatoes on this plant.

The second misdiagnosis borrows calcium from blossom-end rot, usually as eggshells, garden lime, or a dose of Epsom salt that is not even calcium. Alabama is explicit that blossom drop is unrelated to any nutritional disorder, and no source here treats a calcium deficiency as a cause of it. Alabama does warn in the other direction: too much of any fertilizer raises salt levels around the plant, and high salt levels can cause blossom drop. Dumping amendments on the problem is not a neutral act. Advice that belongs to blossom-end rot should stay with blossom-end rot.

What actually helps

Shade is the intervention with real numbers behind it, and the numbers arrive with a setting attached. In commercial mid-Atlantic field trials reported by University of Maryland Extension in 2015, shade cloth draped over staked tomato rows once the plants began to set fruit raised yields in the shaded rows by an average of 30 percent, with quality and size up significantly against the same varieties grown unshaded. In a cooler season with good rainfall, the gain was about 15 percent. Read the accompanying figure carefully: the shaded rows in it were on white plastic mulch and the unshaded rows on black, so mulch color rides along with the shade.

Maryland is cautious about its own result. The timing is narrow, because shade cloth laid over plants during vegetative growth decreases growth, and waiting too long after fruit set begins forfeits the benefit to fruit quality. Maryland does not think growers should go out and cover all of their tomato fields with shade cloth, and says plainly that this row method does not work inside a high tunnel, where the whole structure has to be covered rather than the rows inside it. Maryland's heat-stress guidance points to 30 percent black shade cloth.

Purdue's shade guidance is written for high tunnels, which Maryland has just told us is a different system. Inside one, Purdue reports that studies found 15 to 50 percent shade cloth reducing temperature by only 3 to 4F at times when light intensity was high, and suggests a moderate shade level, 30 percent or less. Above 50 percent shade, Purdue writes, plants developed larger but thinner leaves, longer internodes and less vegetative biomass, and studies suggest heavy shade might reduce sugar content through a lower photosynthesis rate. Shade buys a few degrees. It does not manufacture a cool night.

Water evenly. Alabama says to avoid overwatering, sets the need at about 1 to 1.5 inches of water weekly, and warns that insufficient watering or drought stress can cause blossoms to abort. University of Minnesota Extension's practice note is that watering early in the day is better, with an exception that matters in exactly this weather: if plants are wilting or showing other signs of heat stress, water them as soon as possible, even in the heat of the afternoon or evening. Do not save a wilting plant for tomorrow morning. Be clear about what watering buys, though. It does not cool a flower. It removes the second cause of dropped blossoms so heat is the only thing left to fight.

Go easy on fertilizer. Alabama: too much nitrogen fertilizer can cause excessive vegetative growth, resulting in blossom drop or delays in flower development. Missouri says the same, that high nitrogen pushes the plant toward vegetative growth at the expense of fruit set. Alabama adds that too much of any fertilizer raises the salt levels around the plant, and those salt levels can cause blossom drop by themselves. The classic picture is a big, dark, thriving plant carrying almost no tomatoes.

Then wait. Alabama: once temperatures become more favorable, the plants will produce flowers and set fruit. Maryland: most tomato cultivars can make up for this setback of low fruit production with increased flower formation once it cools a bit. Do not expect it to be instant. Maryland's line during a heat wave is that the week will have repercussions over the next few weeks, and since the damage lands primarily at pollen development, about nine days ahead of the open flower, it is reasonable to expect fruit to trail the cool spell rather than arrive with it. That last step is our inference. No source we found puts a number on the recovery lag.

Heat-set varieties, and the honest limit

The category is real and it has a date. Alabama Extension writes that commercial growers in the southeastern United States suffered from blossom drop for many years until the advent of heat-set tomato varieties in the 1990s, and describes them as bred to tolerate the high day and night temperatures expected in the summer and early fall. Many of them, Alabama adds, set fruit well under other stresses too, its example being extended cool and rainy periods rather than more heat.

These are a few of the names with a university or a breeder behind them rather than catalog copy. Solar Fire came out of the University of Florida's Gulf Coast Research and Education Center in Bradenton. Announcing it in January 2004, its breeder called it the best bet yet for a tomato that can set fruit at warm temperatures, a claim about Florida's commercial crop that is now more than twenty years old. Alabama Extension names Bella Rosa, Phoenix, Red Bounty, and Tribute as heat-set varieties that have performed well in Alabama even during extended periods of hot weather, and in a separate note lists Florida 91, Redline, and Sun Leaper. Maryland Extension names Solar Set, Sunmaster, and Sunchaser among hybrid determinates, and Arkansas Traveler, Homestead 24, and Hazelfield Farm among heirlooms, hedging that these are a few but certainly not all the tolerant varieties bred to perform better in the heat. Maryland's planting advice elsewhere is heat-tolerant varieties like Heatmaster, Florida 91, and Celebrity.

One of those sits in this catalogue: Celebrity, which Maryland names as heat tolerant. A second, Super Sioux, is adjacent rather than heat-set. South Dakota State University Extension raises it in a sentence about cultivar differences in resistance to both heat and cold, calling it a non-hybrid cultivar developed in Nebraska that has good resistance, without saying to which. SDSU does not call it a heat-set tomato, and neither will we. The rest of the names above stay on this page rather than in the catalogue, because we have not profiled them.

If your favorite is an heirloom, the odds are worse, but this is a tendency and not a rule. Alabama notes that heritage and heirloom varieties tend to be more sensitive to the conditions that induce blossom drop than modern varieties, and suggests planting the heirloom soon after the last frost so it fruits ahead of the worst of the summer. Maryland's tolerant list carries three heirlooms on it, so tend to is the right reading.

No source here claims immunity, and neither do we. Purdue's read of the research is that exposing plants to three hour periods of temperatures above 104F on two successive days may cause fruit set failure. SDSU states it shorter, that as few as three hours at 104F can stop fruit set altogether, dropping the two-day condition Purdue keeps. Both statements are made about tomatoes, with no exemption carved out for the heat-set lines. The word extension services use for those varieties is tolerate, and it is the right word. A heat-set variety widens the window. It does not remove the ceiling.