Advice on fertilizing and irrigating conventional vineyards is often easy to come by, but the rules change in a bonsai vineyard where you plant grape vines in pots and trim the roots every year. You must supply water and fertilizer to keep the vines healthy and to get a good crop. The key question is how much.

Grapes are famous for growing in poor soil, and it’s easy to over fertilize. The excess vigor can encourage vegetative growth at the expense of fruit and make the vine hard to manage. Over watering can be a problem too, especially late in the season when the vines are ripening fruit. Excess water will find its way into the grapes, leaving the grower with great looking plump fruit that is unsuitable for making wine. This is why growers often withhold water late in the season.

Vines in a conventional vineyard can send their roots deep into the ground in search of nutrients and water, so they’re almost never cut off the way a potted vine can be. My approach is to always water with a dilute solution (about a tenth of concentration recommended for outdoor plants) of complete fertilizer, like Miracle-Gro, then water according to where we are in the season and what the vine needs. When the vine is growing and demanding water, it gets more fertilizer. Late in the season after most growth is finished and the vine is preparing for dormancy, it gets much less. This simplifies the problem by reducing it to one question, “how much water does the vine need?”

Unfortunately, I can’t give a one-size-fits-all answer like this many gallons (or liters) per week. I usually start with a tentative schedule, like watering (until the well drained soil is thoroughly wet and just starting to drip through the drainage holes) once a week, and watching for signs of water stress to see if I need to supplement. Are the leaves and tendrils strong or limp? Does the soil dry out very quickly? These signs would push me to provide more water. If it’s early in the season, I have a bias toward more water. Later in the season I might tolerate some water stress to avoid over watering. The ripening fruit gives clues too. Firm bulging grapes look terrific, but don’t make great wine and indicate too much water. If the fruit starts to shrivel, like raisins, then you’re at the other extreme of too little water. If you know how to listen, your vines will tell you.

In yesterday’s post, I mentioned that my Pinot Noir clone, ESP374, is not highly regarded. Its biggest problem is that it sets more fruit than it can ripen well. It over-promises and under-delivers, and, ever the optimist, does the same thing year after year. To make good wine from this clone, I pluck out immature grape clusters until there are no more than the vine can ripen. It’s called cluster thinning, and most vines can benefit from it. But how do you know what the right number of clusters is?

There’s the expensive way: you can hire someone else to tell you. There’s the cheap way: you can ask another grower in your area. There are also rules of thumb published in books and on the internet. All of these answers will be for vines, planted in conventional vineyards, that are free to grow their roots as far down as they like. In fact, the answers will usually be in terms of tons/acre (or tonnes/hectare). What about all of us suburbanites growing grape vines in pots? We trim the roots and are left with vines that can produce less than if they were planted in the ground. So even if we could convert tons/acre into pounds/pot, we would still be over cropping. It looks like we’re on our own.

I can’t explain this glaring and puzzling oversight, but here’s how I deal with it. I control the hight of the vine for my convenience; I need to be able to reach the top easily for pruning, harvesting, applying bird netting, and so forth. I look for opportunities to create a spur, and prune each spur so that it’s long enough to push at least one fruitful bud. I suppose I’m relying on the vine to push the right amount of foliage for it’s root system, and managing the buds and clusters so that all the grapes ripen well. I thin to insure that there is only one cluster and at least fourteen leaves per shoot. If I find that some shoots don’t push out enough leaves, I’ll aim for two shoots per cluster next season. To do that I would thin one shoot normally, then remove all clusters from a neighboring shoot. The goal is to make sure that there are enough sugar-producing leaves to ripen all the grapes.

I’m making some changes for my Swenson Red, as I mentioned in this post. As I learn more, my approach will evolve.

In yesterday’s article about pruning, I mentioned that I pruned to spurs on a vertical cordon. That means the trunk is permanent and I maintain short branches of one year old wood, called spurs, that come out of the trunk. If those one year old branches were long, a dozen buds or more, they would be called canes. Canes would make repotting the vines more tedious and would require more support. That’s why I decided to spur prune.

It turns out that some varietals need to be pruned to canes in order to bear fruit. In those varietals, the buds closest to the trunk are not fruitful. If you spur prune them, you’ll get lovely foliage but no grapes. That’s what happened to my Swenson Red last year, and may happen again this year. I pruned in March, and had a vague idea that I should be less aggressive, but I didn’t really know what went wrong or how to fix it. When I prune for next season, I’ll know what to do, but this season, I’ve got longish spurs that aren’t quite canes.