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What can cause tomato flowers to drop before blooming even once?

They never open at all. So, there's no chance for external pollination. They get to the point to where they're almost about to open before they fall off.

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    One is from the seed of an early girl tomato. The other one is a red currant tomato (probably domestic) of an unknown indeterminate variety. The currant tomato does have some open blooms on it, but some fell off before blooming, too. – Shule Nov 22 '14 at 10:05
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One cause: Excessive heat will do it, we had a 1 week hot spell back in July that knocked out tomato production for quite a few gardeners here.

Fortunately for my garden, we had part of the crop flower early enough and the other flower late enough that we didn't get hurt much. The ones that tried to flower during the extreme temps lost the flower buds and tried again a little too late in the season.

Diversity of varieties planted paid off really well.

More research also gives two other causes. Cool weather can cause the blossoms to drop off after they've been pollinated, and excess nitrogen can cause excessive growth with no blossoms or blossom drop.

In extreme overapplication, nitrogen can be a secondary cause of blossom end rot, the primary cause being free calcium lack in the soil.

Test the soil for NPK and acidity. For fertilizer, don't go much above 7-9-5 and fertilize after fruit has set. Burpee notes that 2-3-1 applications from time to time are necessary as Tomatoes are heavy feeders and need the nutrients. So, more light applications over time or time release will be less likely to cause issues as you're dosing nutrients out as the plant needs them.

Tomatoes need phosphorus and appropriate application of calcium in the order of Lime if the soil is too acid or Gypsum if the soil is in the proper pH range.

After testing to see if you have too much nitrogen in the soil, removing excess nitrogen involves using several methods, grow early season crops like broccoli, cabbage or other leaf vegetables that aren't harmed by excessive growth, later season heavy nitrogen feeders like corn or till in carbon rich organic materials so the soil bacteria use up the free nitrogen in digesting it.

We can get the soil right, the weather will unfortunately do what it darn well pleases.

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    That's a good answer that works and I'll upvote you for it, but my tomatoes aren't overly hot. Do you think the following answer about peppers that I just found would also apply to tomatoes? plantvillage.com/posts/… In that case, I did use some 24-8-16 fertilizer. Maybe that was the problem. If so, would more phosphorus and potassium to compensate help, or just make the environment more toxic? Would just waiting until it used the nitrogen (without adding more fertilizer) help? – Shule Nov 21 '14 at 23:34
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    @Shule That's a strong fertilizer. Tomatoes like a ratio where the P is highest. Can you switch fertilizers and see what happens? – J. Musser Nov 22 '14 at 1:42
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    Yep, too much nitrogen can kind of cause them to overgrow leaves and stems. After doing a little research, I find it also causes blossom drop. Excessive concentration in the soil around their roots also causes root burn. As @J.Musser says, they need phosphorus and also calcium. If your soil has proper acidity, use Gypsum (calcium sulphate) if your fruit shows blossom end rot. In practice, there's no such thing as balanced fertilizer. Soil test, find out what concentrations the plant likes and amend to what the soil needs. Test again in a year or two to see how you've shifted things. – Fiasco Labs Nov 22 '14 at 2:12
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    @J.Musser I can switch fertilizer. I just need to figure out which one for the phosphorus. [@Fiasco Labs I didn't know gypsum was calcium sulphate. That's good to know. Thanks for the advice.] We already have some rose flower food, which is much higher in both P and K (equal amounts of each), and much lower in N (but it still has a little N in it, if that's a concern, considering what may be the current levels). I don't know the exact ratio. I'm open to other suggestions. – Shule Nov 22 '14 at 10:23
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The commonest cause for blossoms falling, particularly if they drop from the knuckle, is dryness, both at the root and in the air. Hot, dry air has already been mentioned in one of the other answers as a possible issue. Lack of pollination also causes this problem, so increase humidity and water, and if the flowers remain and do open, tap them gently occasionally to encourage pollination - this simple solution might solve the problem. If it doesn't then looking at the growing conditions (you haven't said how you're growing them, how much root room they have, etc) and then nutrient supply are next on the list, in that order. Adding extra nutrients won't be very useful if the plants don't have enough root room or aren't getting enough water, because they won't be able to take up feeds properly.

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