Spring pruning of grapes: when, how many buds to leave and whether “bleeding” harms

1Why prune in spring

Grapes bear fruit on shoots that grow from last year’s cane, so every year the vine needs fresh cane and the old and surplus cane has to go. Pruning does three jobs: it maintains the shape of the vine, selects the cane that will carry the crop and determines how much crop the vine can carry.

Pruning in spring is more convenient than in autumn for two reasons. First, you can see which buds survived the winter. Second, universities such as Iowa State and the University of New Hampshire recommend late winter or early spring for the main pruning. New Hampshire warns that early winter or autumn pruning can increase winter injury. For the autumn stage, when you only remove what is clearly surplus and leave a reserve, read the article “Autumn pruning of grapes”.

2When to prune

• After the hardest frosts, before bud break. According to eXtension, the permitted window is from leaf fall to bud break, but waiting until the end of the coldest days lets you assess injury and adjust the load. For Iowa this time is from late February to early April. For Ukraine the dates depend on the region: go by the weather, not the calendar.
• In dry weather and without hard frost. In frost the cane is brittle.
• Cover varieties are pruned after the cover is removed, once the danger of hard frost has passed. If you partly pruned them in autumn, in spring what remains is to finally correct the load. How to uncover the vine is described in the article on winter cover.
• Do not leave it until the buds open. New Hampshire advises tying fruiting canes to the wire before the buds swell, because afterwards they break off easily.

3Step 1. Check the buds after winter

Before cutting, look at whether the buds are alive. Texas A&M AgriLife gives benchmarks for grape growers: if fewer than 15% of buds died, pruning need not be changed; if 15–35%, leave 35% more buds; if 35–50%, twice as many; if more than 50%, ordinary dormant pruning may not be worthwhile and badly injured vines are best not pruned in a hurry. This is especially relevant for varieties that in your area work at the edge of winter hardiness (see “No-cover grape varieties”).

A practical way to check: with a sharp blade, cut across several buds from different parts of the vine. A live bud is green inside, a dead one is brown or black. This is common practice, not a norm from a document.

4Step 2. Choose the cane

Keep:

• one-year cane (grown last summer), evenly brown and firm;
• about as thick as a pencil (8–10 mm, per Penn State Extension): a very thin one is weak, and very thick “vigorous” shoots can be less frost-hardy;
• close to the base of the arm: eXtension advises keeping spurs as close to the arm as possible.

Remove old, diseased, broken and unripe cane, as well as cane that grows into the vine and crowds it. Remember: the berries will be carried by the shoots that grow this spring from the buds of last year’s cane. The cane you keep from last year is the future crop, and cane that has already fruited is removed.

5Step 3. Work out the load

A common pruning mistake is not that you cut “wrong” but that you left too many or too few buds.

• Too many buds. According to eXtension, shoots crowd the canopy and shade one another, and the vine may not ripen a big crop; at a very high number of buds the shoots become shorter and carry fewer clusters.
• Too few buds. The vine responds with excessive growth: shoots become more vigorous with many lateral shoots, the canopy is shaded again and the fruit ripens worse.

How to count. Weigh the cane you cut from the vine. The balanced-pruning formula [20+10] from eXtension goes like this: 20 buds for the first pound (≈0.45 kg) of cane and 10 for each additional pound. For example, if the pruned cane weighs 3 pounds (≈1.4 kg), you leave 40 buds. For some wine varieties (Chardonnay, Riesling and others) in the mid-Atlantic United States the formula [20+20] is recommended. eXtension also notes that formulas are less reliable if you trim shoots in summer. This is a guide, not a law: stronger vines carry more, weaker ones less.

Only buds with clearly defined internodes are counted; basal buds (right at the base) are not. For comparison, New Hampshire describes a mature vine with four fruiting canes: 10 buds on each, plus a two-bud renewal spur near each.

Judge the vine by eye as well. If last year’s shoots grew strong but not excessive, the load was right. Weak shoots: leave fewer. Over-vigorous ones: you can leave more, if there is room on the trellis.

6Step 4. Form the fruiting unit

The classic option is a fruiting cane plus a renewal spur:

• The fruiting cane. The long part of the cane that carries this season’s crop.
• The renewal spur. A short shoot of 2 buds near the base: next year it will produce new shoots, and in this way the vine does not move away from the trunk.
• Keep the units as close to the arm as possible, otherwise over the years the fruiting positions “creep” outward.

7Step 5. How to cut

• Sharp, clean pruning shears; wipe the blades after a diseased vine.
• The cut is smooth, 2–3 cm above the last bud, best in the middle of the internode.
• Try to make small cuts, because large wounds on old wood heal poorly.
• Tie the cane to the wire before the buds swell.

8“Bleeding”: does it harm

If clear liquid runs from the cuts after pruning, this is the vine “bleeding”. eXtension, Iowa State and New Hampshire write that it is a normal process and there is no evidence of harm to the vine. The sap moves under root pressure when the soil warms to about 7–9 °C (45–48 °F) and the roots wake up. It contains sugars, amino acids, minerals and plant hormones (cytokinin and gibberellins), and possibly also helps clear air bubbles from the vessels after winter.

What this means for you:

• Do not panic. Pruning in late winter or early spring is often accompanied by bleeding, but it does not harm the vine.
• Do not change the timing because of bleeding. eXtension does not advise shifting pruning just for this.
• Tie the cane to the wire before the buds swell, even if the vine is bleeding.
• In dry years the vine may not bleed at all: it depends on soil moisture.

The only exception the sources mention is field grafting: excess sap can interfere with the union of rootstock and scion. Ordinary pruning is not affected.

9Two-step pruning

Where late spring frosts occur, two-step pruning may be useful: in winter the cane is shortened only partly (left longer than needed), and at the start of bud break it is brought to the target number. According to eXtension, this delays bud break of the basal buds by a few days. The method also lets you assess injury once more.

10The most common mistakes

1. Cutting too early(in autumn or early winter) with no reserve: frozen buds leave no choice.
2. Pruning frozen cane:it is brittle and breaks.
3. Not checking the budsand cutting to plan without taking the winter into account.
4. Leaving too few buds “so the vine doesn’t get tired”The result is rampant growth and later ripening.
5. Leaving too manyAn overloaded vine ripens worse and overwinters worse next winter.
6. Not tying the cane in timeAfter bud break the shoots break off easily.
7. Pruning a young vine to a mature vine’s schemeYoung saplings need quite different pruning in the first years: see the separate article “How to train a young grape sapling”.

11Frequently asked questions

12Sources

• Pruning Grape Vines: An Overview, eXtension (Grapes): timing, balanced pruning [20+10], the effect of too many and too few buds, two-step pruning.
• Grapevine Bleeding, eXtension (Grapes): what vine bleeding is, soil temperature, no evidence of harm, field grafting.
• When is the best time to prune grapevines?, Iowa State University Extension: timing in Iowa, bleeding does no harm.
• Pruning and Training Grapes in the Home Vineyard, University of New Hampshire Extension: timing, tying before the buds swell, schemes for a mature vine.
• Dormant Cane and Spur Pruning in Bunch Grape Vineyards, Penn State Extension: cane thickness, reserve of buds.
• Bud cold hardiness, Texas A&M AgriLife Extension (Helwi, Scheiner, 2018): assessing bud injury and adjusting the load.

Formulas and thresholds from English-language sources are given as guides for your vines; for a particular variety and region the load may differ. Pounds and degrees Fahrenheit have been converted to metric units and rounded.