In most instances, grape growers should purchase planting stock from nurseries or commercial propagators. Occasionally, however, it is necessary or desirable for growers to produce their own vines. For instance, it might be desirable to increase a new cultivar before a supply of plants is commercially available or to produce replacements for missing vines in an established vineyard. Growers should be aware that many new cultivars are patented, and propagation may be restricted.
Cultivars of grapes, like many other fruits, can be reproduced or propagated asexually. Grapes do not grow true from seed; that is, the seedlings will not be genetically identical to the cultivar that produced the seeds. Most grapevines are reproduced by hardwood cuttings or by layering of canes. These methods of asexual propagation ensure that the plants are genetically identical to their parents. In some instances, scions of healthy cultivars are grafted upon specific rootstocks. Cuttings or scions should always be taken from vines known to be true to name. Methods of asexual propagation are described in the following sections.
The most common way to propagate grapes is by hardwood cuttings. Almost all commercially grown cultivars are easy to propagate from cuttings. Cuttings should be made from well-matured dormant canes of the preceding year’s growth. The preferred cane size is 1/4 inch to 3/8 inch in diameter with 4- to 6-inch internodes.
Cuttings usually are made in late fall or early winter. Each cutting should contain three to four buds, although two-bud cuttings are satisfactory in mist or greenhouse propagation. Make the basal cut just below the lower bud, and the upper cut 1 to 2 inches above the top bud. Make cuts so that the upper and lower ends of the cutting can be easily identified.
The cuttings may be sorted into uniform lengths and bundled for convenience in handling. Place the cuttings in cold storage (32°F to 35°F). Cuttings can also be stored by burying them in a well-drained trench and covering them with up to 3 inches of soil until spring. Buried cuttings should be mulched with 8 to 12 inches of straw for protection against severe cold.
As soon as the soil can be worked in spring, remove cuttings from storage or the trench and plant them in nursery rows (Figure 16). The rows should be located on deep, well-drained, fertile soil that is in a good state of tilth. Space rows 3 to 4 feet apart and make a furrow 6 to 7 inches deep. Set cuttings vertically in the furrow about 5 inches apart and firm soil with the top bud just above the soil surface.
It is critical that the cuttings are set in the same direction that they were growing so that the basal end of the cutting is down and the distal end is up. The polarity must be maintained for the cuttings to root. Cuttings also can be planted through black plastic mulch to help control weeds and to retain heat and moisture.
As the season advances, shoots will develop from the above-ground bud, and roots will develop from the nodal regions below ground. Vines should be maintained in the nursery row in a high state of vigor during the growing season. Control of diseases, insects, and weeds in the nursery row is critical. Regular watering and application of fertilizer will ensure adequate growth. New vines will be ready for planting in the vineyard early the next spring. To compensate for cuttings that do not survive, start about twice as many cuttings as vines required.
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| FIGURE 16. Grapevine propagation in the field (nursery). |
To produce new vines for spring planting during the same year, hardwood cuttings can be rooted during late winter under a mist system in the greenhouse (Figure 17). This method saves one year in the propagation of new vines over conventional outdoor rooting methods.
For greenhouse propagation, take cuttings from healthy vines of the desired cultivars in early December before any major winter damage has occurred to the wood or buds. Make two- to four-node cuttings as described in the previous section, then tie the cuttings in bundles, wrap in damp burlap or place in polyethylene bags, and store at 32°F to 33°F.
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| FIGURE 17. Grapevine propagation in a greenhouse mist bench. |
In early February, cuttings are removed from storage and inserted into a suitable rooting medium in the greenhouse (Figure 17). Vermiculite or peat/perlite is excellent for rooting because of their freedom from weed seeds and diseases. Sand or mixtures of sand and peat are also satisfactory.
Before insertion into rooting media, the lower ends of the cuttings can be dipped in a commercial rooting hormone such as indolebutyric acid. However, rooting hormones are generally not needed. A flat or greenhouse bench should be filled with media to a depth of 6 to 8 inches. Cuttings are planted so that the lower cut and node are pushed down to near the bottom of the flat or bed with the upper bud extended just above the media surface. Space cuttings 1 to 2 inches apart in rows 2 to 3 inches apart.
As soon as the cuttings are in place, mist them intermittently to maintain a high and constant relative humidity during the rooting period. A mist system that operates automatically for approximately 6 seconds every 6 to 10 minutes during the day is satisfactory. The duration and frequency of the mist may need to be adjusted as the cuttings begin to grow shoots and to account for sunny or cloudy conditions. The mist can be turned off at night.
Bottom heat provided by heating cable under the flats or in the bottom of a bench hastens rooting. Rooting is most satisfactory if day temperatures in the greenhouse are maintained between 65°F and 70°F and night temperatures around 60°F.
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| FIGURE 18. Potted grapevine progagated from hardwood cutting in the greenhouse. |
Usually, the cuttings develop roots and leaves within four to six weeks (Figure 18). At this time, the rooted cuttings can be transplanted into one- to two-gallon pots for easy transplanting to the field later. A suitable mixture for filling the pots is 1/3 peat, 1/3 sand, and 1/3 soil. Another alternative is a premixed commercial potting soil. Steam sterilization of the soil mixture before filling the pots prevents weed growth and diseases.
After the rooted cuttings have been potted, place them back under the mist for a few days to allow the roots to become established in the new medium. Once established, the plants should be moved to a conventional area in the greenhouse. Moderate temperatures of 70°F to 75°F encourage growth of new vines.
The vines must be watered regularly and receive weekly applications of a dilute fertilizer solution to maintain growth. The fertilizer solution should contain nitrogen, phosphorus, and potassium. Minor elements are also desirable, depending on the potting mix used. Slow-release fertilizer mixes are also available and can be incorporated into the soil before transplanting.
Plants are ready for setting in the vineyard when new shoots are about 12 inches long. If plant growth becomes excessive in the greenhouse, lower temperatures must be used to slow growth and harden the vines before they are taken to the field. During the hardening-off period, maintain night temperatures at 40°F to 45°F and day temperatures at 65°F to 70°F. Regardless of the state of growth, new vines should not be transplanted until all hazard of frost is past. Greenhouse-grown plants are extremely sensitive to frost.
The entire process also can be carried out in individual pots or elongated rooting containers by using peat and perlite or a commercial potting mix and placing the cuttings directly in the containers under mist as described earlier.
Regularly inspect cuttings in the greenhouse for insects and diseases. Whiteflies and powdery mildew can be troublesome pests. To eliminate these pests, use an appropriate pesticide according to label directions.
All grape cultivars can be propagated by layering. This method is used primarily for replacing missing vines in established vineyards. However, it is too cumbersome for production of large numbers of plants.
Layering is done in late winter or early spring. Vigorous one-year-old canes are used. The canes remain attached to the mother plant. This supports the establishment of a well-rooted plant during the first season. Lay the canes in a shallow trench dug in the desired location of the new vine. Place a two- to three-node section of the cane at the bottom of the trench. At least two distal buds should extend above the soil surface. Cover the part of the cane in the trench with 3 or 4 inches of soil and tamp firmly.
Roots normally develop from the covered nodes in a few weeks. Leaves and new shoots will develop from exposed terminal buds. During the growing season, any shoots developing between the layered area of the cane and the mother vine should be removed.
New plants produced in a vacancy in the vineyard are left in place, and the connecting cane is cut off the following spring after the new vine is well established. If the layered vine is to be moved, it should be dug and transplanted after one year’s growth.
Grafting allows growers to propagate grape cultivars on a special rootstock, such as one resistant to certain root parasites. The European (vinifera) grape, for example, is highly susceptible to a destructive insect pest known as grape phylloxera or root louse. Consequently, cultivars of this species cannot be grown on their own root systems in phylloxera-infested soils. Therefore, it is necessary to graft European cultivars on phylloxera-resistant rootstocks to grow them successfully on many U.S. soils and those in other parts of the world. Experience indicates that rootstocks with high phylloxera resistance are also resistant to certain parasitic nematodes.
The use of resistant rootstocks for European cultivars is more important than for American and French hybrid grapes, with some exceptions. Therefore, most U.S. vineyards have been established with own-rooted vines, especially Concord. The root systems of these cultivars apparently carry considerable tolerance to phylloxera and other soil-borne parasites. However, differences in tolerance have been found in some American grape cultivars. Delaware, for example, has performed better on tolerant rootstocks than on its own roots.
Where vigor of own-rooted vines is characteristically low, favorable results can be expected from using resistant rootstocks. Poor vine vigor and productivity often occur on sites where old vines were removed and the vineyard replanted. In replant situations such as this, resistant rootstocks may prove a distinct advantage with American cultivars or with French hybrids.
Among the rootstocks carrying high resistance to phylloxera, nematodes, and possibly other soil-borne parasites are Couderc 3309, 5BB, and SO4. These rootstocks are conventionally propagated by cuttings or layering. Once rooted, the stocks can be used for grafting to any desired cultivar.
Bench grafting is commonly used in propagating grape cultivars (especially vinifera) on special rootstocks. This operation is conducted indoors and involves grafting of single bud scions onto rootstock cuttings (Figure 19). Grafting is done in early spring before growth starts. Once callusing is complete, the grafted cuttings are planted into the nursery.
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| FIGURE 19. Bench grafting: Grafting machine with grafted and waxed cuttings. Insert: Omega cut graft. Scion (left); disbudded rootstock (right). | FIGURE 20. Side whip grafting technique. |