Growing Grapes in New Hampshire [fact sheet]

Introduction

Low winter temperatures and a short growing season generally limit grape production to the southern half of New Hampshire. Grape production in northern New Hampshire is limited to extremely well-protected sites with short-season varieties and, even under the best of circumstances, success is limited. That said, if you have an appropriate site, Northern-grown grapes make excellent fresh eating, juices, jellies, raisins, and wine. A small home vineyard with even just a vine or two can be a beautiful and productive addition to the landscape, yard or patio. This fact sheet provides the basic information you need to successfully grow grapes in NH.

Site and soil

The ideal vineyard site provides full sunlight, protection from prevailing winds and freedom from late spring frosts. Plant grapes on a southern slope for best results. Grapes will grow in a wide range of soils but do best in a deep, well-drained sandy loam with a pH of 5.3 to 6.0. Clay soils can be made more suitable for grape production with the addition of sand and organic matter (aged or composted manure, compost, peat moss or a green manure crop).

Soil testing can be done through a number of private and public labs. UNH Cooperative Extension offers this service. Forms and instructions are available online, or you can call our Infoline at 1-877-EXT-GROW (1-877-398-4769) or email answers@unh.edu.

Prepare the vineyard site a year ahead of planting to allow time to adjust the soil pH, build fertility, and eradicate perennial weeds like quackgrass. Plant bare soil to a fall cover crop such as spring oats (2 to 3 pounds of seed per 1,000 square feet) by early September to provide protection against soil erosion during the fall and winter.

Establishing the vineyard

Plant vigorous one-year-old bare-root plants in early spring, about May 1. Handle young grapevines carefully to prevent their roots from drying out. If they cannot be planted immediately, keep plants in cold storage until planting time.

Space plants 8 feet apart in rows 8 to 10 feet apart. Set plants at about the same depth they were planted in the nursery. Be sure to pack soil firmly around the roots, and water in to settle the soil around the roots. Immediately after planting, prune vines to a single cane and head back to 2 buds. During the first growing season, tie all shoots that develop loosely to a stake to keep them growing straight and upright. (Figure 1.)

Potted plants may be planted at any time throughout the growing season – if planting after June 1, do not prune the newly planted vines. For nursery-grown plants, it is especially important to keep them well-watered after planting. As a rule of thumb, be sure to provide the equivalent of 1 inch of rainfall per week if nature does not provide it.

The grape trellis

Grapevines require support, which is most often provided by a trellis. The trellis must be strong and substantial enough to carry the weight of the vines plus a heavy crop during strong winds.

It is important to choose your training system before you build the trellis. A typical trellis consists of firmly-set, well-braced posts at intervals of 10 to 15 feet along the row. Attach two strands of No. 9 or No. 10 wire to the posts – one at 3 feet and one at 5-6 feet above the ground. Set end posts at least 3 feet deep leaving 6 feet above ground. End posts should be well-braced or angled outward to prevent pulling by the taut wire coupled with the weight of the grapevines. (See Trellis End Post Assembly Designs for Vineyards for more details about trellis design).

Figure 1: The planting year. A newly set plant (left), immediately after pruning (center), and several weeks after planting (right).

Pruning and training

There are countless named pruning and training systems for grapes. See ‘Pruning and Training Grapes in the Home Vineyard’ for a detailed discussion of training systems suited for cold climate grape-growing.

In colder climates, cane-pruned systems are often better than cordon-pruned systems because they are less susceptible to winter injury. Of the cane-pruned systems, we’ll describe just one: the four-arm Kniffin. This system is well-adapted to vigorous and trailing varieties. It uses a two-wire trellis, as described above.

The planting year: Prune the newly set plant back to two strong buds. Allow all shoots to grow, and tie them loosely to a stake.

One year after planting: Prune back to 1 or 2 strong upright stems, which will become the trunk(s) (Figure 2).Having two trunks is insurance against winter injury. Tie one or both trunks to the top wire. If they are not long enough to tie to the top wire, tie to the bottom wire and extend a new shoot to the top wire the following spring.

Two years after planting: Select 4 vigorous canes for the arms. Select 2 for the top wire, and 2 for the bottom wire, as shown below. Choose canes that form just below the point where the wire and trunk meet. Prune each cane/arm to 10 buds in length and tie them loosely along the wires. For each of the 4 arms, a two-bud-long renewal spur should be left on the trunk to produce fruiting canes for the following season. Remove all other canes.

Each dormant season thereafter: Replace the arms with canes and leave new renewal spurs. This annual pruning will help ensure consistent production and high fruit quality and will keep the vines at a manageable size and shape.

Prune only in late winter or early spring, since early winter or fall pruning can promote winter damage. Vines pruned in late spring just prior to bud break will “bleed,” but this does not harm the vine. Fruiting canes should be tied to wires before buds begin to swell because buds are easily rubbed off once growth begins.

Figure 2: The year after planting, just before dormant pruning (left), and after dormant pruning, where two canes have been chosen to become trunks (center).
On the right, two years after planting, the two trunks have reached the top wire, and four canes/arms have been selected: two on the middle wire, and two on injured.

These ‘Mars’ table grapes are growing in southern NH, at the NH Agricultural Experiment Station’s Woodman Horticultural Research Farm.

Managing the vineyard

Fertilizing grapes

In the planting year, apply 1/4 pound of 10-10-10 garden fertilizer (or its equivalent) by scattering it in an 18-inch circle around each plant 3 weeks after planting. Double this amount each year until year 3, when 1 pound of 10-10-10 (or its equivalent) is applied per year. Fertilizer should be applied in late April- early May.

Controlling weeds

Complete weed control around grape vines is essential to ensure adequate vine growth and high yields. Frequent shallow cultivation or hand-hoeing is the most common strategy. An organic mulch such as wood chips or sawdust will also help suppress weed growth and conserve soil moisture. However, mulches can also delay ripening of the grapes in the fall and may encourage rodents that can damage vines.

Harvesting

Do not harvest grapes until they are fully mature. Color is a poor index of maturity since many varieties change color long before they are fully ripe. Almost all varieties become sweeter and less acid as they mature.
Tasting an occasional grape is a good method of determining when grapes are ready. Another good maturity index is seed color, which changes from green to brown as grapes mature. The danger of fall frost, bird or raccoon pressure, or rot and cracked berries may force an early harvest in some seasons.

Insects and Diseases

Problems with several diseases and insects can cause excessive fruit and fruit quality losses in most years. This Pocket Guide for Grape IPM Scouting in the North Central and Eastern United States is a very useful resource that has photos of most grapevine problems. The most common problems in NH home vineyards are described, and shown in photos, below.

Insects

Japanese Beetle (Figure 3) – This beetle has one generation per year, and can cause severe defoliation of many plant species including grapevines during July and August. Management options include hand-removal and judicious use of labelled insecticides.

Hornets – Various hornets and wasps are attracted to cracked and overripe fruit, and can make harvest difficult.
Harvesting during the early morning in cool weather and before fruit crack or become overripe can minimize problems.

Phylloxera – A soft-bodied insect forms galls or protrusions on the underside of leaves; primarily with Vitis vinifera
varieties or hybrid cultivars. Remove affected leaves; chemical management is usually not warranted.

Tumid Gall Wasp (Figure 3) - The grape tumid gallmaker is a tiny midge (a type of fly) that lays eggs on developing leaves, stems and fruit clusters, causing galls that can interfere with fruit development and yield. The tiny larvae are found within the galls. Chemical management is usually not warranted.

Figure 3. Two common insect pests of grapevines: the Japanese beetle (left), and galls formed by the grape tumid gall wasp (right).

Diseases

Black Rot (Figure 4) – The fungus that causes black rot causes leaf spots and fruit rot. Black rot is more likely to occur in wet years, and in plantings in partial shade.

Downy Mildew – This disease damages the leaves and canes, and occasionally the fruit. It usually appearing mid-summer if conditions are wet.

Powdery Mildew – This fungal disease affects leaves and fruit, and is favored by dry conditions.

Anthracnose (Figure 5) and Phomopsis – These fungal diseases affect canes and leaves.

Figure 4. Black rot is probably the most severe fungal disease of grape. Typical lesions on the leaves (left) and on the unripe fruit (right) are shown.

Figure 5. Anthracnose is another common fungal diseas of grape in the home vineyard. It causes pitting on the stems (left) and shothole lesions on the leaves (right).

Varieties

Most grape cultivars are not hardy except in southern New Hampshire on sites where winter temperatures do not fall below -10 to -15 ° F. The varieties listed below should do well on good sites throughout southern New Hampshire, with those marked as “hardy” possibly tolerating winter temperatures down to -15 to -25°F. Some varieties contain seeds (“seeded”) where others are seedless. For varieties described as “Slipskin”, the skin readily separates from the pulp of the berry.