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. 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 32F to 33F.
In early February, cuttings are removed from storage and inserted into a suitable rooting medium in the greenhouse. 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 65F and 70F and night temperatures around 60F. Usually, the cuttings develop roots and leaves within four to six weeks.
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 70F to 75F 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 40F to 45F and day temperatures at 65F to 70F.
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.
Layering:
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 years growth.
Grafting:
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 vigour of own-rooted vines is characteristically low, favourable results can be expected from using resistant rootstocks. Poor vine vigour 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 soilborne 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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