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Old July 18, 2021   #1
venturabananas
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Default Soil temperature and blossom drop?

Can high soil temperatures lead to blossom drop?

I grow most or all of my tomatoes in pots, usually 20 gallon pots with 2-3 plants per pot. The pots are black plastic and the soil in them really heats up on sunny days.

I've always read that high temperatures can lead to blossom drop, failure to set fruit. I had always assumed the problem was related to air temperature, not soil temperature. I'm fortunate to live in a place (coastal California) where a typical summer day seldom even reaches 80F and nighttime lows are in the low 60's. From what I've read, this temperature range should be ideal for fruit set. Yet during mid-summer, for a couple of months, my plants make lots of blooms and drop them all. They set fruit earlier in the season, in late spring, and later, in fall.

This year I decided to start measuring the soil temperature in the pots. By early afternoon, the soil temperature is 95-100F on a typical summer day. First thing in the morning, it is in the high 60s.

My best bet is the high soil temperatures during the day, or the high fluctuation in temperature, in the root zone is stressing the plants and causing blossom drop. The plants are otherwise healthy. They don't wilt and they grow well.

In support of the soil temperature hypothesis, this blossom drop seems to only happen on plants in pots fully exposed to the sun all day. One plant in a pot that is shaded in the afternoon doesn't drop blossoms, nor does another one planted in the ground. But that could be coincidental.

Do any of you have any definitive information or experience on whether high or fluctuating soil temperatures can cause blossom drop?
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Old July 18, 2021   #2
PaulF
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Good question. After a little research all I can find is the word, "temperature." With soil temps getting higher I would guess there is a temperature sink right around the pots and even the air temperature at the pot is higher than away from the pot.

There are several reasons for blossom drop besides air temperature. Stress is another main reason. Root stress probably adds to blossom drop problems. Do you have a thick mulch on the surface of the pot or is it bare soil? That may help keep soil temp down. Shade cloth may also help.

I think you have found the reason for blossom drop but now you just have to find a solution. The two I have mentioned could be a good start. Moving the pots to a better location if possible may be another solution. I am sure there will be more suggestions.
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Old July 18, 2021   #3
venturabananas
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There is a reasonably thick bark mulch, but I think it is mainly the black plastic of the pot heating in the sun. Some folks who grow figs in pots paint the pots white and that makes a big difference in soil temperature. I'm going to try that, or wrap something around the pot that shades the black plastic sides.

I did find an article that mentioned soil temperatures over 95F by midmorning causing blossom drop. I don't think I actually have that high a temp that early in the day, but certainly by later in the day I do.
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Old July 19, 2021   #4
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Hot pot will affect the good function of the roots. Worse nutrition can lead to more blossom drop. Temperature swings will certainly add to that. I cover my pots in thick alu foil.
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Old July 19, 2021   #5
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I grow a lot of cherrys.Seems like once the blossom shows you have about two to five days before it dies and drops.You have a couple of hours in the morning for the dry pollen to escape either by buzzing wings,wind,and artificial manipulation,electric toothbrush etc.I tap my plants with a stick.If the pollen is damp due to the humidity it will not move.I live in a 10 B,Really humid condition.We do pollen checks by a black cardboard square held underneath a blossom and tapped to see if anything falls out.
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Old July 19, 2021   #6
zeuspaul
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Not specific to blossom drop I shade my black plastic pots when convenient. I use white floor tile leaned against the sunny side of the container because it is what I have available.
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Old August 16, 2021   #7
venturabananas
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I ended up wrapping my pots in reed privacy fencing -- looks kind of natural and fits in with the more tropical theme of my yard. It dropped the daily high temperature of the soil in my pots about 15-20 degrees F on sunny days -- with a high around 80. I'm getting a bit more fruit set, but still lots of blossom drop. I hung a little outdoor thermometer from one of the tomato cages and it is registering mid 80's in the shade during the warmest part of the day and low 60's at night. I would have thought that range would be OK for fruit set. So ??
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Old August 16, 2021   #8
zeuspaul
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That sounds like a good solution especially since it looks good! If I were to do something like that it would provide a pathway for little destructive critters.
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Old August 17, 2021   #9
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Are the plants vigorous, or kind of spindly? The plants will blossom drop once they think they have enough fruit set for the current conditions (leaf area and color intensity are pretty much directly correlated with the fruit production), not much you can do about that.
A bit of extra boron might help also.
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Old August 24, 2021   #10
venturabananas
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Quote:
Originally Posted by zipcode View Post
Are the plants vigorous, or kind of spindly? The plants will blossom drop once they think they have enough fruit set for the current conditions (leaf area and color intensity are pretty much directly correlated with the fruit production), not much you can do about that.
They are reasonably vigorous. Certainly there are enough healthy leaves to support more fruit than they are holding.

They did this last year, too, and then started to set fruit better once it cooled down a bit -- hence my focus on temperature. I just find it odd, given that I live in an area with a very mild climate (e.g., yesterday's highs and lows were 74 and 55 degrees F). But the roots and flowers themselves do experience higher temperatures, due to being in pots and sitting on a concrete driveway.
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