Vine-ripened grapes are sweet, juicy and a bit crunchy. Although northern gardeners expect to be denied the pleasure of cultivating their own grapes, grapes can be grown in climates where winter temperatures plummet to -40C. Choose hardy varieties like early-fruiting, dark purple Valiant or white Kay Gray. Although these varieties may also be grown outside with some success, they flourish under cover.
Positioning the Grape Growing Greenhouse
Vine rows should be oriented to optimize uninterrupted sunlight reception. In northern Canada, planting rows from slightly north/east to south/west can result in better vine growth. Vines catch the first early morning rays of sunlight along the full length of the rows as the sun rises.
Photosynthesis reaches maximum levels in the morning, decreasing toward afternoon as light intensity decreases, making it important to take advantage of first light in areas with shorter day length. The greenhouse should be built to accommodate this row orientation.
Grape Growing Greenhouse Design
Construct the unheated greenhouse using a narrow lean-to design so that the long north wall is taller than the long south wall. A distance of about 7 or 8 feet should be allowed between the walls to provide enough growing distance along a raised trellis as well as ample space beneath for tending the vines.
The south wall should be low, about 4.5 feet tall, with the roof angling upward toward the north wall at about a 40 – 45 degree angle. This steeply sloped roof design acts somewhat like a solar collector.
If possible the north wall should be constructed of a solid material like wood or brick to keep out cold north winds. If necessary this wall can also be insulated using styrofoam rigid insulating material. The main door will also be built on the north wall.
While sidewalls may be made of regular clear greenhouse plastic, use milky-colored twin wall polycarbonate sheets for the roof. The sheets let in plenty of light but not the strong burning sunlight that is admitted by clear products.
This is important because the vines will grow up high in the greenhouse close to the roof. The double wall also adds a degree of insulation.
Pollinating Greenhouse Grapes
Because most grapevines are pollinated by wind, design the greenhouse with tightly screened ingress windows positioned so that prevailing winds can be carried into the greenhouse to stir the leaves.
Corresponding egress windows should be built into the opposite side of the greenhouse. Self-propelling fans can be installed in windows on both sides and motorized if additional assistance is necessary. Using heat-activated window openers simplifies exhausting the greenhouse if overheated.
Grape trellising can be as simple as slipping long heavy wooden dowels across the rafters holding up the roof but whatever method is used, keep the trellis strong and high up in the greenhouse following the angle of the roofline. Several support posts may be necessary as the vines age and become thicker.