Growing grapes in Iowa means pruning in late winter and harvesting in the fall. But there are a few other tasks to do in summer to produce high-quality grapes.
This includes sucker removal, thinning, shoot positioning, and supplemental irrigation.
When winter pruning backyard grapevines, we generally leave 40 to 60 buds per plant, but this does not mean they will only have 40 to 60 shoots when they grow in the summer. If the cultivar you are growing is not adapted to your region, it's possible you will lose some buds due to low winter temperatures and have fewer shoots. More likely, however, is that you will have an abundance of shoots.
Early summer is a great time to remove excess shoots because you can do most of it by hand, without the use of pruners. The removal of shoots on the trunk of a grapevine is generally referred to as suckering. The shoots on the trunk (suckers) often will not have fruit and will end up being shaded out (Figure 1) from the grapevine canopy above it. All the suckers should be removed in early summer unless you need to leave one to replace a trunk.
Shoot thinning in the canopy is important as often grapevines produce more shoots than is ideal. Generally, we want to leave around 4 to 6 shoots per foot vine. If our plants are taking up about 8 feet of trellis, that means we want to leave around 40 to 50 shoots on them. In Figure 2 are shoots that were removed from the same vine on the same day. The shoot on the left is vigorous, has long tendrils, and large flower clusters. As you move to the shoots on the right, they are less vigorous, lack tendrils (or have small ones), and have fewer and smaller clusters. When shoot thinning in the grapevine canopy, our goal is to remove the shoots that do not have much fruit or are low in vigor.
Shoot thinning should be done in early summer when the shoots are small and can be removed by hand. An additional benefit to shoot thinning early in the summer is that it increases the amount of sunlight that gets into the middle of the grapevine canopy (Figures 3 and 4). This sunlight is not only important for fruit ripening, but it also helps the grapevine buds to potentially become more cold hardy for the winter and have more fruit on them in the following season.
Shoot Positiong & Combing
When summer officially begins in late June, grapevine shoots are growing rapidly. If the vines do not have a large crop on them, this rapid shoot growth can persist throughout the growing season. Timely management of these shoots is important.
Early in the season, grapevine shoots are not strongly attached to the vine, so trying to manipulate them at that time can lead to breakage. Additionally, when shoots are manipulated early in the season the short shoots are not heavy enough to stay in place after strong winds.
If you wait until mid-summer to move them around, the shoots are too long to effectively manipulate, and the tendrils start to grab the trellis and other parts of the vine. When the shoots are 2-3 feet long, around late June, it is generally the ideal time to position the shoots in Iowa.
For vines with cordons (permanent wood) trained horizontally to a high wire, we want to orientate the shoots to grow downward, rather than allow them to grow horizontally on the trellis (Figure 5). When the shoots grow horizontally on the trellis, they will continue to shade the fruiting zone as they grow throughout the season. This can reduce the quality of fruit this year, and the shoots that are shaded tend to be less cold hardy in the winter and have shoots with fewer clusters on them the following season.
Positioning the shoots downward is often referred to as ‘combing’. While you can position each shoot individually, most growers will pull down handfuls of shoots growing horizontally on the trellis and position them downward. Try to get the shoots evenly distributed on both sides of the vine. Figure 6 shows that once the shoots have been combed, portions of the cordon and fruit clusters are now exposed to sunlight at the top of the canopy. If you wait until mid-summer to comb vines, the clusters can become sunburned (Figure 7) since they developed in the shade too long.
Grapevines require one inch of water per week, either from rain or irrigation, for good growth and crop production. Irrigate plants weekly during hot, dry weather. When watering water the entire root zone (not the foliage). A spot sprinkler or well-positioned soaker hose can work well when supplemental irrigation is needed.
Learn more about growing grapes in this publication: Growing Grapes in the Home Garden