Tomatoes have flowers but no fruit

This happened with my Green Zebras last year, and is happening with my Paul Robesons this year. Tons of big fluffy yellow flowers, and then nothing. Just little dried bits where the flowers used to be. Not a single fruit.
I live in southern Utah, zone 7 or 8. Daytime temps are currently around 90-100F. Direct sunshine during most of the daylight hours. The tomatoes are planted in raised beds with a mix of my native clay soil and some commercial potting soil. I water them fairly deeply every three days or so. I gently tap the plants every few days to help pollinate the flowers. I have applied no fertilizers to the plants, but they have a thin layer of dead grass (cut after it was dead) as mulch underneath. They are planted in the same bed as beans (there were no beads in the Green Zebra bed last year, though).
Other varieties of tomatoes seem to be fruiting okay, in similar growing conditions.
Can someone tell me what is going on, and if there is a way to get them to fruit? --S.
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<http://forums.gardenweb.com/forums/load/okgard/msg0609224625626.html
1) If your tomatoes are caged, trellised or staked, grasp the cage, trellis material or stake, and gently shake the plant. This can loosen up clumping pollen and cause it to move around in/on the flower and pollinate. This works best if you do it during cooler early morning temperatures, or cooler evening temperatures, especially shortly after sunrise or shortly before/after sunset. Doing this a couple of times a day really does help improve fruit set, especially on the occasional "cooler" day.
2.) If your temperatures are on the borderline of those that allow pollination--max. temp. of about 92 to 95 and min. temp. lower than 75 (but higher than 55), shaking alone may not be enough but you can improve the odds by "cooling" down the pollen and flowers with a brief spray of water from the hose during the hottest part of the day (or night). You don't want to hit the plant with such a hard stream of water that the flowers fall off, obviously, though! Do this at least twice daily when temperatures are right at the threshold of being "too hot" and it can cool the flowers just enough that they will set fruit.
3.) Some people use an electric toothbrush (I've never tried this one myself) to "vibrate" or "shake" the flowers to improved fruit set. If you are interested in trying this, there is a thread on this topic on the Tomato Forum that describes how it is done.
4.) Some tomato experts recommend that, on certain plants like Brandywine which is notorious for not setting fruit in hot weather, you emasculate the Brandywine flower and use a Q-tip to transfer pollen from a DIFFERENT variety to the Brandywine. You still will get Brandywine fruit from this fertilization, if it takes, but seed saved from THAT tomato could give you a cross-pollinated fruit if you plant that seed next year. If you want to know how to do this, do a search for "emasculate flower" on the Tomato Forum. One of the guys on the forum linked a video that shows how. Emasculating a flower and manually transferring pollen is one way that home gardeners/tomato hobbyists can do deliberate cross-pollination in an attempt to "breed" a tomato plant that is a cross of two or more different varieties.
You can use the above techniques to improve the odds of fruit set any time, but they are especially effective when we are having the occasional slightly cooler weekend.
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Try this:
http://www.gardeners.com/Vegetable-Blossom-Set-Spray/34-444,default,pd.html
Or a similar tomato fruit set spray. One year when my tomato plants were having trouble setting fruit, I bought this and tried it on one plant. That plant had plenty of tomatoes, while the other plants did not.
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Lots of interesting things I hadn't considered. Thanks; I'll try them all. I actually remember spraying my tomatoes a little each day when it was hot a few years ago. Apparently, I forgot all about that! --S.
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It's probably too hot for them. Optimum fruit set occurs with a night temperature range between 60 - 70 degrees F. The plants need the rest from the heat of the day. How hot is it there at night?
If it's too dry then the pollen might not be sticky enough. You might need to spray the foliage with water during the day to help the plants along.
You can also try to help things along by shaking your tomato plants to help distribute the pollen yourself.
marcella
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