1 - Seedless grapes may make seeds.

Yard and Garden - September 11, 2010

Q.

We have our first crop of grapes from two new vines that we planted a couple of years ago. The plants are 'Reliance' and 'Seedless Concord'. We also planted 'Flame' and 'Thompson Seedless' a year later, so we haven't gotten anything from these vines yet. As you can probably guess, we chose these varieties because they are seedless. There are seeds in both the 'Reliance' and 'Seedless Concord' grapes that are now getting ripe. I understand that 'seedless' often means the fruit actually has small soft seeds, but these are normal-sized hard seeds. Actually, some of the 'Reliance' grapes do have small, soft seeds, but most of them are large and hard. From any bunch of grapes, around 2/3 to 3/4 of the grape seeds are large and hard, with the remainder being small and soft. In case it is significant, the grapes are also about half the size I would expect. The flavor is good, but very strong. Is the seed issue a cultural thing? In other words, can I impact the seed size and hardness by changing cultural practices, such as more or less fertilizer, more or less water, or does mulch matter? I currently use small gravel ("crusher fines") as mulch, I spread two tablespoons of 10-10-10 fertilizer around each plant in the spring, and I water with a drip system that is supposed to provide 4 gallons of water per plant each week. I thinned the grapes to allow two bunches on each of three "laterals" (I think that is the right word from the book I used for pruning instructions) per plant. Our home (that is, where the grapes are growing) is at about 5300 feet elevation in very sandy soil, which I enriched with compost at the time of planting.

A.

This has been an interesting question to research. Seedless grapes do have vestigial seeds. 'Seedless Concord' is said to produce seeds in a hot environment. I found nothing like that regarding 'Reliance', however it was stated for all seedless grapes that the climate has an influence on the size and hardness of the vestigial seeds in all of them. The heat this summer may be the culprit, and our location and climate may make this a regular problem. Another consideration is that some seedless grapes form only a few seeds unless they are near a compatible pollinator variety.

The size of the grapes is dependent on the cultivar of grape you are growing, cultural practices (pruning, fertilization, and irrigation), and the production of certain hormones by seeds in the grapes. Commercial grape growers sometimes supply the hormone, gibberellic acid, as a spray to cause seedless grapes to enlarge. Home gardeners rarely do this and have smaller grapes. Your cultural practices seem appropriate, so it may be that the seeds that are forming are not supplying the hormones necessary for larger berries to form.

I grow 'Flame' seedless grapes with good success. The berries are small, probably because my work keeps me away from home when I should be pruning and caring for the grapes. However, they taste great and I have not noticed any objectionable seed in them.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

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