Things that keep a plant from blooming include shade, too much nitrogen fertilizer, heavy pruning, and poor nutrition. If there are no blooms the plant obviously won’t set fruit. But even if a plant blooms it may not produce fruit. A variety of things can happen but here are a few common problems.
Too much nitrogen can knock the blooms off after they form. I have seen this happen on tomatoes, okra and cucumbers but it can happen to any vegetable. It is rare on woody plants but is possible. You have to count every source of nitrogen. I have talked with gardeners who swear they haven't used too much fertilizer. But then you find out they are not counting manure and compost plus they are spraying miracle grow every week. Fortunately, the plants will usually outgrow this problem if you can back off the extra nitrogen.
Another problem I see on cucumbers, cantaloupes, squash and watermelons is lack of insect pollination. These plants require insects to move the pollen. When insects aren't present, the blooms drop off. Female cucumber and squash flowers have an obvious ovary that looks like a small cucumber or squash. So some people think the fruit is falling off when the fruit has never developed. On these plants the male blooms show up first. Some gardeners get worried when these fall off but that is normal.
When we consider edible fruit trees like peach and apples a different set of problems occur more frequently. A late frost will kill these blooms.
Some fruit requires insect pollination while other fruit trees require wind pollination. I haven't figured out any rhyme or reason to which trees do and which trees don't. A new gardener will just have to remember them. Apples, pears, cherries, persimmons, some plums, blueberries, certain muscadines and strawberries require insects. Anything that reduces the number of insects can prevent flowers from becoming fruit. Pesticide sprays, cool weather, and other nectar sources have all contributed to poor insect visits which translate to poor pollination. Peaches, some plums, blackberries, and grapes can wind pollinate without insects although insects often triple and quadruple the yield.
Most of the nuts require wind pollination. Pecans, hickories, chestnuts, and walnuts all produce pollen that must ride the wind to the female bloom. Sometimes a complete crop failure is attributed to heavy rains when male pollen should be floating around.
The figs we grow around here will produce fruit without pollination.
Some fruit trees require two different cultivars to set fruit. We call this cross pollination. Again this is something you have to remember. Apples, pears, some cherries, some plums, pecans, and some blueberries all require cross pollination. It is possible for a cultivar to be pollen sterile. This means you need three different plants for maximum yield. Magness pear is the only pollen sterile cultivar on our current recommendation list. The best nursery catalogs will tell you if cross pollination is required.
Apples will not set fruit if they are sprayed with Sevin a few days after pollinating. The orchard sprays I see on the shelf now don't contain Sevin but some brands used to contain Sevin.
Heavy insect pressure or heavy disease pressure will cause fruit to drop even after it has pollinated. Some apples will normally drop some fruit in June. This may be due to crowding on the spurs. Insect damaged fruit also drops at this time.
In general, fruit set on ornamentals isn't as much of a problem. I get the most questions on hollies. Holly plants are entirely male or entirely female. The males will never set fruit. Most of the female trees need a male within insect range to set fruit. Burfordi hollies are a major exception. They will set fruit without pollination. I am guessing a few other hollies will also. There are usually enough native male American hollies and possomhaw hollies to provide pollination but it doesn't hurt to plant a male to make sure you get fruit.
Right this minute, I can't think of any more ornamentals that have problems setting fruit once you get them to bloom. (The next landscape I notice will probably have a dozen.) Most Beautyberries set fruit with wind pollination. The Japanese beautyberry may be the exception but they will do fine if you plant more than one. Cornus kousa sets fruit without cross pollination. Hearts a busting (Euonymus American) will fruit fine without cross pollination. Our native viburnums didn't set fruit very well this year. That may have been due to frost or to heavy rains that keep the insects inactive.
Article was written by David Goforth Agriculture Extension agent North Carolina Cooperative Extension Cabarrus County Center. Visit my homepage http://cabarrus.ces.ncsu.edu/ or http://cabarrus.ces.ncsu.edu/index.php?page=lawngarden or my blog http://gardeninggurugoforth.blogspot.com/
Contact me at David_Goforth@ncsu.edu. Reviewed 2007.