Where have all the flowers (and fruit) gone?
Bruce Bordelon, Professor, Department of Horticulture and Landscape Architecture, Purdue University
This winter has been one of the coldest on record in the Midwest according to the climatologists. But actually the minimum temperatures we have experience are just a little colder than “normal” for our USDA Plant Hardiness Zone 5b (average minimum temperature -10 to -15˚F). We had one event of -18˚F in Lafayette, another with -14˚F, and a few others at or near -10˚F. One of the most notable differences this year from a “normal” winter is that we’ve had those -10˚F temperatures on more than one occasion, and in one case in early January, the temperatures remained -10˚F or colder for more than 24 hours.
Perennial plants vary in their ability to withstand cold temperatures. Minus 10˚F is the threshold where we start to see damage on more tender crops like peaches and blackberries. Many factors affect the degree of cold damage that may occur. The actual temperature reached and the duration of the cold are the most important. Other factors such as the general health of the plant, the time of season the cold occurs, and the temperatures preceding the cold event all can influence the amount of damage that occurs. It appears that a couple of things may have happened this winter. First, we had a sub zero event in mid December, before plants had reached their full potential for cold hardiness. Secondly, as mentioned, we had an event when damaging temperatures occurred for more than 24 hours. And third, we’ve had multiple potentially damaging events some of which were preceded by relatively mild temperatures. It appears that all those events together have caused considerable damage to flower buds on a number of crops and ornamental plants. That means we won’t see near as many bright yellow Forsythia flowers this spring, nor have a crop of peaches, blackberries or grapes. A recent survey of the overwintering buds of those plants in our area has shown considerable damage. Essentially all the peach and Forsythia flower buds evaluated were dead. About half the grape varieties in our research planting at Lafayette have less than 10% live primary buds, meaning there will be little if any crop on those. Luckily we have several cold hardy grape varieties they have survived the winter very well and should produce a full crop of fruit. The popular backyard variety Concord has about 50% live primary buds, so growers should leave twice as much wood at pruning this year to make up for the loss and assure a good crop of fruit.
It is relatively easy to evaluate flower buds for survival. Simply slice through the bud with a sharp knife or razor blade. If the center of the bud is bright green, then it is healthy. If the center is black or brown, then the bud has been killed by cold temperatures. Since perennial plants set their flower buds the previous season, that means we won’t have any flowers or fruit this year on those that have been damaged.
Click image to enlarge
Figure 1. Grape bud before cutting. Bud scales protect the bud from drying out.
Figure 2. Grape bud showing a live (green) primary bud
Figure 3. . Grape bud with dead primary bud
Figure 4. Peach flower bud before dissecting
Figure 5. Dead peach flower bud
Figure 6. Forsythia buds before dissecting
Figure 7. Dead Forsythia buds
Figure 8. Blackberry bud before dissecting
Figure 9. Blackberry bud dissected. Note bud tissue appears green and healthy. However, in blackberry the vascular tissue that connects the bud to the cane is often the site of damage.
Figure 10. Blackberry cambium showing browning, indicating winter injury.