A selection of fruits from the author’s farm. The darker grapes are Worden, the green ones Edelweiss. Apples, including sweet Golden Russets and even tiny red crab apples, can help jelly thicken. (Barbara Damrosch)

Grapes are among the most plentiful fruits you’ll find at our place. Some are seedless, and others have seeds that you must spit out as you munch. Some varieties are green, others red.

My favorites are Edelweiss and Worden, both seeded and highly flavorful. Edelweiss is a very sweet green variety, and Worden bears clusters that are such a deep blue that they look almost black as they dangle from the arbor. A chance seedling of Concord, it resembles that classic variety but is earlier and hardier. I boil it down into a rich syrup that I either can or freeze, to pour on ice cream or drizzle into an apple pie.

I’ve also made jam from Wordens, but I confess that I find jams and jellies a bother to make. Getting them to jell is tricky, because so much depends on the fruit and its degree of ripeness. Its level of acidity and the amount of pectin the fruit contains play a part, too.

Pectin is a polysaccharide found in the cell walls of plants. As a source of soluble dietary fiber, it is considered beneficial to human health and essential to making jelly jell.

Published lists that rank fruits by their abundance of natural pectin can be confusing and contradictory, although most put lemon peel and apples at or near the top. In any case, cooks tend to rely on purchased pectin, usually in powdered form, in hopes of making the process foolproof. You can buy natural or synthetic pectin for making fruit preserves, and each comes with precise directions. But with the variables being what they are, it’s still a matter of trial, error and luck.

Personally, I’d rather have my jam err on the runny side — and call it syrup — than have it come out too hard. Gummy bears, anyone? Nevertheless, I have learned that adding some apple to other fruits when making preserves will increase their ­jellability.

Forty years ago, when I was living in Connecticut, I had a run-in with runny grape jam, and a neighbor suggested I throw in an apple to help it set up. So for the next batch, I headed to a wild area near the house where old crab apple trees had established themselves many years ago. Feral grapes, the kind called fox grapes, were rampant, twining through shrubs and trees. Most likely those were the native Vitis labrusca, from which many old American varieties, including Concord, were derived.

I gathered the grapes and the marble-size crab apples in my basket, an inauspicious combination, perhaps. The grapes had the musky “foxy” flavor they are known for, and the little yellow apples were even more tart and unpromising. But they looked beautiful in my basket. And the jelly I made with them — with enough honey added — was pleasingly rustic, and lo and behold, it jelled perfectly, with no extra pectin added.

Crab apple trees are extremely popular with home gardeners. They are compact enough for small yards and tolerate city conditions well. They require much less care than “eating” apples — a top-dressing with mature compost in fall and a bit of late-winter pruning will usually suffice. Although a crab is basically the same plant as an orchard apple — and some relatively large-fruited ones were originally used for hard cider — most have been bred and sold for their ornamental value, as they blanket themselves with flowers in spring (adored by bees) and abundant and colorful red or yellow fruits in fall (relished by birds).

A few varieties, such as Dolgo, with red, 11/2-inch fruits, are planted specifically for human consumption, especially in jellies. Centennial, a cross between Dolgo and a normal-size apple called Wealthy, bears two-inch fruits and adapts well to most regions. But because I now have an orchard full of regular dessert apples and pie apples, I’d be more apt to add a couple of sweet Golden Russets to a pot of simmering grapes, on their way to becoming jam.

But a few handfuls of tiny crab apples, from the trees growing next to the grape arbor, would also do fine in a pinch. I’d just add more honey.