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Key to grape success? Deep irrigation

Bunch of grapes on vine in sun
Grapes need a lot of sun and water for best results. (Photos: Debbie Arrington )

Vines require a lot of water, sun for best harvest

Are your grapes getting enough water?

Summer irrigation is crucial for a good harvest. Grape vines may adapt to drought conditions and low water supply, but they’ll bear a lot fewer if any grapes.

According to the UC Cooperative Extension master gardeners, grapes need deep irrigation every two weeks during these hot summer months (or more if the heat is particularly brutal). Give vines a slow, deep soaking that fills their root zone.

Drip irrigation works well with grapes, but remember, they need a lot of water. According to the master gardeners, each mature trellised vine needs 8 to 10 gallons a day. Less vigorous or young vines still need 6 to 8 gallons daily. (By comparison, a tomato plant requires about 5 gallons of water a week.)

Consistent irrigation will prevent several grape problems.

“Avoid water stress during the period between bloom and berry softening,” say the master gardeners. “Fruit on the vine may succumb to cracking if the vines are allowed to dry and then wet again. Check the soil profile to be sure the plants are getting adequate water but not too much.”

Thirsty vines don’t like a feast-or-famine water cycle. For nice plump grapes, keep the soil evenly moist. Use a moisture meter, a soil probe or just look – with a trowel, dig down a few inches.

Grapes require sun for sweetness. Although vines may produce grapes in shady spots, the plant needs at least eight hours a day of full sun for the grapes to produce enough sugar to taste “sweet.”

Grape bunch in shade on vine
Grapes that ripen in shade tend to be less sweet.

Sunlight on the fruit can bring out the grape’s sweetness and color (especially red varieties). That’s why vineyards may snip off leaves shading grape clusters. Grapes ripened in the sun tend to be sweeter than those that ripen on the shady side of the plant. But clusters still need some afternoon shade or the grapes can become sunburned.

When to pick? Grapes ripen on their own schedule and may look ready before they actually are at their sweetest. The only way to tell is test. Pick a plump grape and taste it. If it’s not sweet, wait a few days, then test again.

For loads of great grape advice, check out the UCCE grape page:


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Garden Checklist for week of April 14

It's still not warm enough to transplant tomatoes directly in the ground, but we’re getting there.

* April is the last chance to plant citrus trees such as dwarf orange, lemon and kumquat. These trees also look good in landscaping and provide fresh fruit in winter.

* Smell orange blossoms? Feed citrus trees with a low dose of balanced fertilizer (such as 10-10-10) during bloom to help set fruit. Keep an eye out for ants.

* Apply slow-release fertilizer to the lawn.

* Thoroughly clean debris from the bottom of outdoor ponds or fountains.

* Spring brings a flush of rapid growth, and that means your garden needs nutrients. Fertilize shrubs and trees with a slow-release fertilizer. Or mulch with a 1-inch layer of compost.

* Azaleas and camellias looking a little yellow? If leaves are turning yellow between the veins, give them a boost with chelated iron.

* Trim dead flowers but not leaves from spring-flowering bulbs such as daffodils and tulips. Those leaves gather energy to create next year's flowers. Also, give the bulbs a fertilizer boost after bloom.

* Pinch chrysanthemums back to 12 inches for fall flowers. Cut old stems to the ground.

* Mulch around plants to conserve moisture and control weeds.

* From seed, plant beans, beets, cantaloupes, carrots, corn, cucumbers, melons, radishes and squash.

* Plant onion sets.

* In the flower garden, plant seeds for asters, cosmos, celosia, marigolds, salvia, sunflowers and zinnias.

* Transplant petunias, zinnias, geraniums and other summer bloomers.

* Plant perennials and dahlia tubers for summer bloom.

* Mid to late April is about the last chance to plant summer bulbs, such as gladiolus and tuberous begonias.

* Transplant lettuce seedlings. Choose varieties that mature quickly such as loose leaf.

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