University of Vermont Extension 
Department of Plant and Soil Science

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WHY PLANTS FAIL TO FLOWER OR FRUIT
 
Dr. Leonard Perry, Extension Professor
University of Vermont
 
           
Perhaps your flowers failed to flower, or didn’t last as long as you’d like this season.  Maybe your neighbor's yard is ablaze with color every summer, yet for some reason those shrubs you planted a few years ago still haven't flowered.  Perhaps your shrubs, trees or grapes that should have had fruits didn’t.   If you’re wondering why plants failed to flower or fruit, you’re not alone as this is one of the most common gardening questions.
           
Plants fail to flower for various reasons. Herbaceous plants may have been bred for foliage not flowers, such as coleus and many of the new coralbells.  Some perennials may have been “forced” or manipulated into bloom out of season by the grower so they’ll be in bloom during the May sales season.  The next year this same plant might bloom later in summer during its normal time.
           
One of the more common reasons for bloom failure with woody plants is immaturity or “juvenility”. Trees, in particular, must reach a certain age before they begin to flower. 
           
Even if the plant is old enough, the growing conditions may be too poor to allow flowering. Plants that require full sun, for instance, may grow but fail to flower in the shade.  And, if a plant is too old, as often is the case with perennials, they may flower poorly if at all.  Peonies are an exception to most perennials, not flowering if they are planted with their base of the plant from which stems arise (the “crown”) more than a couple inches deep or under mulch.
           
Cold winter temperatures may kill the flower buds. This often happens to forsythia and some spring bulbs. Unfortunately, nothing can be done to prevent this type of injury except to buy more hardy cultivars (cultivated varieties). Since woody plants generally have separate flower and leaf buds, the latter being more hardy, cold in winter will often kill the flower buds yet the plants leaf out fine.  Or, the flower buds may only open on the bottom, as is common with forsythia—those buds being protected by snow cover.  When choosing fruit trees, if you live in a cold pocket with late spring frosts, plant late-blooming cultivars.
           
Plants that are not fully hardy will be most susceptible to cold injury. This is why it is important to not fertilize woody plants the last half of the growing season, so plants will harden properly for winter. 
          
Improper pruning may cause failure to flower. Some plants bloom only on last year's wood, particularly spring bloomers like lilacs and rhododendrons. Prune these right after bloom, if needed at all.  Cutting the plants back severely or too late removes all the flowering wood for the coming year. This happens with pruning climbing roses improperly.  This might explain, too, why some shrubs (as some of the new hydrangeas) that die back to the ground each winter never bloom—they need to keep last year’s wood and buds alive.
           
Some plants fruit on last year’s wood, such as summer raspberries, and then only once.  So cut the old fruiting canes back in late fall, which allows time for nutrients to recycle back to the roots.  These raspberry canes won’t fruit again if left on the plant.  And, cutting canes back that are only one-year old will remove fruiting wood for the coming year.
           
Overfertilizing with nitrogen can sometimes cause plants to grow only leaves and stems. Such plants, often herbaceous or woody perennials, will be quite large but without flowers.  Yet many of the new annual flowers are bred to require heavy fertility.  With too little fertilizer for such annuals, they may stop blooming by mid summer.  Some annuals just bloom for a shorter period and not late into the season.  You can see online which annual flowers have lasted best through the summer at our Burlington, Vermont All-America Selections display garden (pss.uvm.edu/ppp/aaswp.html).

A plant may fail to fruit because of the same reasons as for failure to flower. If there are no flowers, there can be no fruits. Then again, a plant may flower but fail to have fruits.  One of the most common explanations is lack of proper pollination. Some plants cannot pollinate themselves. They require a plant of the same species but of a different cultivar for cross-pollination.  Plants should be somewhat close (often 50 feet or less) so bees will go between plants.  And the different cultivars should, obviously, overlap in bloom time. Some fruit trees get rather complex as to what pollinates what, information provided in catalogs and online from nurseries.
           
Lack of pollination can occur if cold, rainy weather occurs when a plant is in full bloom. Such weather will keep bees from working, thus reducing or preventing pollination and fruit-set. Or, bees may be more tempted by adjacent flowers and weeds such as dandelions, so not pollinate pear trees for instance if plums are in bloom nearby.  A frost while a plant is in flower also will kill the flowers and prevent fruit-set.  Some woody plants or vines, such as grapes, have another set of buds that will come out if the first is damaged.  So, if the first buds are killed with a late frost, the plant may flower and fruit but just less heavily.
           
Some plants are "dioecious" meaning all the flowers on a plant are either male or female. Both a male and a female plant must be present, and cross-pollination must occur for the female to produce fruit. Examples of dioecious plants are holly, bittersweet, and yew.
           
Some cultivars of fruit trees, in particular, are “alternate bearing” or “biennial bearing”.  This simply means they may bear heavily one year, then take a break the next to recoup their energy with few, if any, fruit.  Bosc pears, and Golden Delicious or Fuji apples, often have alternate bearing.
           
While all these reasons for not flowering or fruiting are based on either the plant or its culture, another may just be wildlife.  Serviceberries, elderberries, currants, blueberries and the like may be picked by the birds and seem to disappear overnight. Deer may have pruned back your perennials, as they do with my garden phlox, so they never bloom if feeding keeps up through the season. 


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