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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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Dig In: Garden Checklist

For week of June 4:

Because of the comfortable weather, it’s not too late to set out tomato and pepper seedlings as well as squash and melon plants. They’ll appreciate this not-too-hot weather. Just remember to water.

* From seed, plant corn, pumpkins, radishes, melons, squash and sunflowers.

* Plant basil to go with your tomatoes.

* Transplant summer annuals such as petunias, marigolds and zinnias.

* It’s also a good time to transplant perennial flowers including astilbe, columbine, coneflowers, coreopsis, dahlias, rudbeckia, salvia and verbena.

* Let the grass grow longer. Set the mower blades high to reduce stress on your lawn during summer heat. To cut down on evaporation, water your lawn deeply during the wee hours of the morning, between 2 and 8 a.m.

* Tie up vines and stake tall plants such as gladiolus and lilies. That gives their heavy flowers some support.

* Dig and divide crowded bulbs after the tops have died down.

* Feed summer flowers with a slow-release fertilizer.

* Mulch, mulch, mulch! This “blanket” keeps moisture in the soil longer and helps your plants cope during hot weather.

* Thin grapes on the vine for bigger, better clusters later this summer.

* Cut back fruit-bearing canes on berries.

* Feed camellias, azaleas and other acid-loving plants.

* Trim off dead flowers from rose bushes to keep them blooming through the summer. Roses also benefit from deep watering and feeding now. A top dressing of aged compost will keep them happy. It feeds as well as keeps roots moist.

* Pinch back chrysanthemums for bushier plants with many more flowers in September.

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