We have an extremely hot summer this year, and we’re finding our grapevine’s leaves are starting to drop. While it’s likely due to the heat (or lack of water), I wanted to do more research to be sure we know what’s causing it. Here’s what I found.
Grapevines are deciduous plants, so they normally lose all of their leaves in the fall and winter. However, if their leaves are falling off in the spring or summer, it’s usually from improper watering, climate, nutrients, pests, and diseases. Ideally, only water when the soil is dry and provide compost and mulch.
So, grapevines shed leaves for several reasons, but how can we tell which issue is causing it, and from there—how can we fix it? Let’s take a closer look.
If your grapevine is losing its leaves in the fall or winter, know that this is normal!
Since grapevines are deciduous plants (like apples, cherries, and peaches), they normally go dormant during the colder months and shed all of their leaves, fruits, and flowers.
The reason why grapevines go dormant is to ensure they survive the winter. Rather than using nutrients and energy to keep leaves alive and fend off the cold, they enter a low-energy, inactive state (similar to how bears hibernate).
By conserving energy during these months, they’re able to use this stored energy and explode with growth once spring arrives.
However, grapevines (and other deciduous plants) need to reach a specific amount of chill hours to stay in dormancy.
Chill hours are every hour that’s under 45ºF and the ideal amount for grapevines is around 600 hours. By reaching this amount, grapevines have the best growth and fruiting potential.
Results of our study demonstrated that the chilling requirements of all three cultivars were adequately reached at 600 hours or more.Effect of Chilling and Photoperiod on Budbreak in Three Hybrid Grape Cultivars, American Society for Horticultural Science
If the weather warms up for an extended period during the winter, grapevines will think it’s spring and wake from dormancy to begin growing again. This poses a problem as a later frost can damage the new growth and stunt the grapevine for a year or more.
But what if it’s not autumn or winter? What causes grapevines to lose their leaves in the warmer months?
2. Improper Watering
Grapevines that lack water have symptoms including leaves drooping, curling, drying, browning, and dropping (usually in that order).
This is a typical reaction of plants when they don’t have enough root moisture to send to their leaves. It’s also made worse when the plants need water the most, usually in hot weather (more on this later).
The best way to water grapevines is to only water when the first 2-4 inches of their soil is dry. You can check this by pushing a finger in the soil. The goal is to have soil moisture similar to a wrung-out sponge.
Make sure you’re soaking the soil at least 2 feet deep as over 90% of the grapevine’s roots are found at this depth.
Also, apply 2 inches of compost and 4 inches of mulch under the grapevine’s drip-line, especially if its soil dries quickly.
Compost provides valuable nutrients and increases the soil’s water retention. Every 1% increase in the soil’s richness or organic matter leads to an additional 20,000 gallons of water held per acre (source).
Mulch reduces evaporation, regulates soil temperature, and prevents soil erosion. As grapevines evolved in forests as a vining understory, they’ve had access to plenty of mulch in the form of fallen leaves and branches.
Reapply compost every 1-2 months and mulch every 3-6 months. Keep these materials at least 3 inches from the grapevine’s main (terminal) stem as they can introduce mold.
Since there are so many factors when watering plants (drainage, slope, plant age, etc.), and every day brings different weather and levels of evaporation, there’s no one volume of water to use.
However, by only watering when the soil is dry, you’re preventing both under-watering as well as over-watering.
You can tell if your grapevine is over-watered if its leaves are drooping, yellowing, or dropping early, and the soil is staying sopping wet for 1 day or more. If the soil is starting to smell like a swamp, it’s likely been waterlogged for too long and root rot is starting to develop.
The good news that is over-watering is easy to prevent if you only water when the soil is dry.
The bad news is poor drainage makes this difficult.
No matter how good you are about watering, if your grapevine’s soil has poor drainage, it will likely become over-watered.
Poor drainage is common in clay soils as clay particles are finer than sand and easily become compacted (see the graphic below for a soil size comparison).
However, clay soils aren’t all bad as they contain the most nutrients.
|Doesn’t hold nutrients well
|Holds nutrients well
|Holds the most nutrients
|Even worse drainage than silt
Because of this, the best soil type for most plants (including grapevines) is loam, or a mix of all three soil types. That way you have the optimal amount of nutrients, water retention, and drainage.
To find out if your soil has poor drainage, perform a percolation test. You can do this by digging a 1-foot by 1-foot hole nearby and filling it with water. If the soil drains slower than 2 inches per hour, it has poor drainage.
When doing a percolation test, keep these two things in mind:
- Make sure to dig the hole outside of the drip-line of the plant to avoid damaging its shallow roots
- 2 inches per hour is just a guideline, not a rule
If you find your grapevine’s soil has poor drainage, amend it with 2 inches of compost every 1-2 months. Over time, the larger particles in the compost work their way into the soil, promoting more drainage (and nutrients).
On the other hand, if your grapevine’s soil has fast drainage (over 2 inches per hour), the solution is still the same.
This is because compost increases the soil’s organic matter, amending both poor and fast drainage. In fast-draining soils, I also recommend providing 4 inches of mulch under the plant’s drip line. However, avoid mulching in poor-draining soils as it can trap moisture and make the issue worse.
For potted grapevines, the soil shouldn’t be staying sopping wet for more than 1 day. Because potted grapevines have their roots confined, digging them up and providing fresh potting soil won’t cause as much transplant shock as it would for planted grapevines.
As an example, my potted Kaffir lime tree had poor drainage and root rot once. But after repotting it with fresh potting soil, it made a quick recovery!
3. Extreme Weather
Grapevines grow best in USDA hardiness zones 4-10, but some varieties such as Valiant can be grown down to zone 2.
Cold weather isn’t too much of a problem for grapevines as they tolerate temperatures -20ºF and below (the average minimum temperature for zone 4).
However, hot weather is a different story.
When the weather is 90ºF or hotter, the grapevine’s leaves begin to overheat and burn, leading to drooping, curling, drying, browning, and dropping.
In times of too much heat (and too little water), grapevine plants can quickly die.
For best growth and fruiting, keep grapevines within 55ºF to 80ºF if possible.
To help keep grapevines cool, it’s important to know how they do this in the first place.
Grapevine’s primarily cool themselves by sending moisture from their roots to their leaves and through a process called transpiration.
Similar to how humans release moisture when they exhale, plants do the same through transpiration.
In his book, The Hidden Life of Trees, professional forester Peter Wohlleben shares his findings that while the top side of leaves is used like a solar panel for photosynthesis, the underside of the leaves is used for breathing and transpiration.
If you’ve ever walked through a dense canopy or forest, and felt an increase in humidity, it’s because the plants are breathing!
So, planting grapevines near other plants and in high-density planting, not only provides the grapevines with some shade but helps cool them through transpiration. These are two reasons why companion planting grapes with other plants is so beneficial.
For example, grapes and oak trees are great companion plants. The oak tree provides the grapes with partial shade and a living trellis to climb, while the grapes attract pollinators and provide mulch for the oak tree with its fallen leaves.
However, as grapevines are commonly grown in hot and dry (Mediterranean) climates, keeping them cool can be challenging.
Tips for Hot and Dry Weather
- Apply 2 inches of compost and 4 inches of mulch. Not only do these materials provide plenty of nutrients, but they keep moisture in the soil—preventing evaporation while promoting drainage!
- Provide partial shade, especially from the hot, afternoon sun. You can use umbrellas, shade sails, or other trees and companion plants.
Mark Shepherd, on his 100+ acre farm in Wisconsin, is a master at companion planting oaks, chestnuts, berries, grapevines, and more to achieve the maximum amount of yields with the minimum amount of work. Check out one of my favorite videos of his below.
4. Improper Nutrients
A lack of nutrients causes deficiencies, which result in conditions such as leaves drooping, curling, discoloring, and dropping.
For example, here are some common deficiencies of grapevines (and their symptoms).
|Entire leaf is pale or yellow
|Dark green veins, rest of the leaf is yellowing
|Broadly pale leaves, foliage color looks mottled or smeared
On the other hand, excess nutrients chemically burn the grapevine’s roots, leading to discolored and dropping leaves. This is caused by over-fertilizing with chemical fertilizers (as compost isn’t potent enough).
The two main ways to fertilize grapevines are with chemical fertilizers and compost.
While chemical fertilizers have nutrients in quantity, they often lack quality. For example, chemical fertilizers block the nutrient exchange between plants and their beneficial soil life, turning healthy soil into dead soil (AKA dirt).
Without beneficial soil life such as mycorrhizal fungi, grapevines have dramatically reduced water retention, nutrient capacity, and pest and disease resistance.
Mycorrhizal fungi promote many aspects of plant life, in particular improved nutrition, better growth, stress tolerance, and disease resistance.Department of Biology, University of Fribourg, Switzerland
The good news is that compost not only promotes beneficial soil life but has more than sufficient nutrients for grapevines. No wonder why so many gardeners are finding compost is replacing their chemical fertilizers.
Whichever option you choose, if you’d like to see my recommendations for fertilizers and compost, check out my recommended fertilizer page.
Nutrients aren’t everything though.
Without a balanced soil pH, plants are unable to properly absorb nutrients from the soil. Many plants require a slightly acidic pH as it dissolves the nutrient solids in the soil.
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
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.
To measure your soil’s pH, I recommend using pH strips or a meter. I prefer using a 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 your grapevine’s soil is too acidic (under 5.5), provide alkaline amendments such as wood ash, charcoal, or lime. On the other hand, for alkaline soil (above 6.5), provide acidic amendments including sand, peat moss, and coffee grounds.
5. Pests and Diseases
|Common Pests for Grapevines
|Common Diseases for Grapevines
|Grape Berry Moths
|Armillaria Root Rot
Grapevine leaves discolor and fall off due to pests and diseases such as Grape phylloxera, Japanese beetles, leaf spot, and root rot. Treat pests by using organic insecticides or companion plants, and diseases with organic fungicides.
You can generally tell if a grapevine has pests by inspecting the leaves and the fruit. You should be able to see the pests themselves or signs of the pest such as holes in the leaves or fruits.
On the other hand, diseases are typically shown as yellow, red, or brown spots or blotches on the leaves and other parts of the vine.
To see a full list of the diseases grapevines get and how to treat them, check out this resource by Michigan State University.
It turns out our grapevine was dropping leaves from the hot weather as well as a lack of water. Once we provided more compost and mulch, and some partial shade from the afternoon sun, it made a quick recovery!