Our grapevine provides us with table grapes and shade by growing along the wooden overhang above the patio. So, when we saw that its leaves started curling, we became concerned. To help solve this, I did some research. Here’s what I found.
Grapevine leaves curl from improper watering, climate, nutrients, as well as transplant shock, pests, and diseases. Ideally, only water grapevines when the top 2-4 inches of soil is dry, grow in USDA zones 4-10, and provide compost and mulch.
So, while grapevine leaves curl from several causes, how can we tell which issue is causing it, and from there—how can we fix it? Let’s take a closer look.
You can tell when grapevines are under-watered if their leaves begin to droop or curl to retain moisture. If not watered for an extended period, their leaves will dry, brown, and drop.
The best way to water grapevines is to only water when the first 2-4 inches of soil is dry. You can check this by pushing a finger into the soil, near the main stem. The goal is to have soil moisture similar to a wrung-out sponge.
When watering, make sure to soak the ground at least 2 feet deep as over 90% of the grapevine’s roots are found at this depth.
But many times, providing a sufficient amount of water isn’t enough. Since grapevines are commonly grown in hot and dry climates (Mediterranean), the soil moisture evaporates quickly.
As a result, grapevines often require near-constant watering to avoid curling, browning, and dropping leaves (especially in the summer).
This is where compost and mulching come in.
Compost not only provides valuable nutrients but increases the soil’s richness. With every 1% increase in the soil’s richness or organic matter, it can hold an additional 20,000 gallons of water (source).
On the other hand, mulch reduces evaporation, regulates soil temperature, and prevents weeds and erosion.
Pro-tip: Apply 2 inches of compost and 4 inches of mulch under the grapevine’s drip line. Reapply compost every 1-2 months and mulch every 3-6 months. Keep these materials at least 3 inches from the main or terminal stem as they can introduce mold.
By using both compost and mulch, the soil can significantly retain more water—making your job easier!
However, there are times when it gets so hot that the grapevine’s leaves burn no matter the amount of water in the soil.
2. Heat Stress
Grapevines do best when they’re grown in USDA hardiness zones 4-10. However, there are some varieties such as Valiant that can be grown down to zone 2.
Frost isn’t too much of an issue for grapevines as they can withstand temperatures down to -20ºF, but the same isn’t true for heat.
While many grape varieties can grow up to zone 10, they do best when the temperature is under 90ºF. Any hotter, and they won’t be able to cool themselves effectively.
Pro-tip: Generally, for best growth and fruiting, keep grapevines within -20ºF and 80ºF.
Grapevines cool themselves in two primary ways: root moisture and transpiration.
Root moisture is fairly common sense, as the plant’s roots transport moisture from the ground to the rest of the plant to keep it cool (including the leaves), but what’s transpiration?
Much like humans, plants breathe and release moisture when hot. For plants, this is called transpiration. But when the climate is too hot and dry, transpiration and root moisture can’t effectively keep up to cool the plant and its leaves. As a result, the grapevine’s leaves droop or curl, and then dry, brown, and drop.
Of course, there will be times when the temperatures exceed 80-90ºF. So, aside from watering, what can we do to help keep our grapevine cool?
Tips for Hot Weather
- Apply 2 inches of compost and 4 inches of mulch. I already mentioned this, but these are the two key practices to avoiding under-watering and heat stress!
- Provide partial shade, especially from the hot, afternoon sun.
Grapevines evolved as a vining plant in forests, so they prefer wrapping their vines around large trunks such as oaks. Contrary to popular belief, the oak tree won’t get strangled.
And both plants benefit from this relationship! The oak gets extra nutrients from the grapevine (and the wildlife it attracts) and the grapevine gets a living trellis and shade from the oak.
Keep in mind that oak trees aren’t the only companion plants for grapes. Many vineyards are finding great results by adding cover crops below the grapevines.
Every year a cover crop is planted in our best vineyards in Napa. A cover crop is comprised of many different plants that are strong in different micro-nutrients that grapevines might need and that the soil might be deprived of after the previous growing season.Grgich Hills Estate
Okay, so you’ve properly watered your grapevine and it’s not getting over-heated, but it’s still getting curled leaves. What’s the next thing we should explore?
3. Lack of Nutrients
The best way to fertilize grapevines is to provide fertilizer as directed or apply 2 inches of compost every 1-2 months. However, many chemical fertilizers have negative long-term effects and waves of gardeners are finding out that compost can replace fertilizers.
When grapevines lack nutrients, their leaves curl, yellow, brown, and drop. This is most common when they’re lacking phosphorus and potassium—two of the three main nutrients plants need (the other being nitrogen).
Fortunately, the majority of fertilizers and compost contain all of the primary and secondary nutrients grapevines need.
To see which fertilizers and compost I recommend, check out my recommended fertilizer page.
Also, if you’d like to diagnose any other specific deficiencies, feel free to reference this table I put together.
|Entire leaf is pale or yellow
|Dark green veins, rest of the leaf is yellowing
|Broadly pale leaves, foliage color looks mottled or smeared
Insufficient nutrients are most often caused by:
- Poor soil
- Imbalanced pH
Poor soils are common with bad soil management practices such as excessive tilling—which exposes the beneficial soil life, drying it out in the sun. This is the quickest way to turn soil into dirt.
Let’s take a more detailed look at the other two main causes of insufficient nutrients.
Nutrient leaching occurs when the nutrients seep too far down into the soil, out of reach of the plant’s roots (beyond about 2-3 feet). This normally occurs when soils have too much drainage or are over-watered.
For example, sandy soils are notorious for leaching. You can see the comparison of sand and other soil particles in the graphic below. Generally, larger particles have less compaction and more drainage.
Grapevines do best with a soil pH of 5.5-6.5. Generally, American grape varieties prefer a pH closer to 5.5 while European varieties prefer closer to 6.5.
While nutrients are important, they’re next to useless if the soil does not have a proper pH. This is because a slightly acidic pH is necessary to dissolve the nutrient solids in the soil and make them accessible for the plant’s finer roots.
Fourteen of the seventeen essential plant nutrients are obtained from the soil. Before a nutrient can be used by plants it must be dissolved in the soil solution. Most minerals and nutrients are more soluble or available in acid soils than in neutral or slightly alkaline soils.Donald Bickelhaupt, Instructional Support Specialist, Department of Forest and Natural Resources Management
Two good ways to check your soil’s pH are either by using a pH strip or a pH meter. I prefer using a pH meter since they’re affordable and easy to use. To see which pH meter I use and recommend, check out my recommended tools page.
If you find that your grapevine’s soil is too alkaline (above 6.5) you can add acidic amendments such as coffee grounds, sand, and peat moss. On the other hand, if your grapevine’s soil is too acidic (below 5.5), add alkaline materials such as wood ash, charcoal, or lime.
Additionally, cold and/or wet soils are not good at promoting nutrient uptake as the grapevine will either be dormant during the cold or too stressed in wet soils.
Aphids are small bugs that suck the sap from underneath the grapevine’s leaves. This loss of sugar and moisture causes the leaves to curl, discolor, and drop. They also deposit honeydew, which attracts ants. If left unchecked, aphids can damage the grapevine’s health and potentially stunt or kill it.
These bugs come in multiple colors including white, yellow, or black, and usually are found hiding underneath the leaves. Typically, aphids won’t cause damage to the fruit, but because they suck sap from the plant, they can compromise its health and therefore reduce fruit size.
The best ways to get rid of aphids and mites on grapevines is by spraying the infected leaves with water or neem oil, or releasing ladybugs (a natural predator of aphids and mites). Most often, a jet of water is enough to get rid of them, but neem oil is a good second option.
For example, when my potted Kaffir lime tree had aphids, I found that a jet of water was enough to blast them off and prevent them from coming back. All I did was remove the hose nozzle and used my thumb to increase the pressure. Keep in mind that too strong of a blast will damage the leaves.
5. Grape Leafroll Disease
Grape leafroll disease is a virus that is transmitted by mealybugs and scale when they feed on grapevine leaves. Symptoms include curled leaves that turn yellow or red while maintaining green veins.
This disease is most common in vineyards, but backyard grapevines can also get it. Typically, grape leafroll disease causes a loss of 30% to 50% of yields.
Currently, there are no treatments or sprays that are effective. However, managing pests such as aphids, mealybugs, and scale can greatly reduce or prevent this disease.