Recently, a reader reached out and asked if I knew why their grapevine wasn’t flowering (or fruiting for that matter). I had an idea, but I wanted to do more research to give them the best answer. Here’s what I found.
Grapevines won’t flower or fruit if they’re not yet mature or have improper nutrients, sunlight, and pollination. In some cases, over-pruning, pests, and diseases weaken grapevines to have poor production. For best results, let grapevines age at least 3 years and provide compost, 6+ hours of sunlight, and pollination.
| Causes of Poor Flowering | Causes of Poor Fruiting |
|---|---|
| Not Mature Yet | Not Mature Yet |
| Lack of Sunlight | Lack of Pollination |
| Excess Nutrients | Improper Nutrients |
| Pests and Diseases | Pests and Diseases |
As you can see, there’s a lot of overlap in the needs for flowering and fruiting. Because of this, merged the two topics and highlighted any text where they differ in the following sections.
So, while grapevines won’t flower or fruit for several reasons, how can we tell which issues are causing it, and how can we fix it? Let’s take a closer look.
1. Not Mature Yet
Like most fruiting plants, grapevines need to mature to at least 3 years old before they flower or fruit sufficiently. While some grapevines flower and fruit sooner, 3 years is the common expected age.
It takes this long because the grapevine’s roots and branches need to be established to gather enough nutrients, water, and sunlight to feed their flowers and fruit.
The exact length of time it takes grapevines to produce largely depends on if it was grafted or grown from seed.
Grafted vs. Grown From Seed
Grafted plants generally grow quickly as they’re a clone of the parent plant, so their DNA doesn’t need to mature. All that’s needed is a root system and foliage. Most fruiting plants that are grafted can start flowering in at little as 1-3 years.
On the other hand, plants grown from seed can take up to 10 years to produce consistent flowers and fruits. This is because they have brand new DNA (much like human children), and not only need to establish roots and foliage but mature as well.
Generally, grapevines purchased from nurseries will be grafted.
But what if your grapevine is over 3 years old? What other issues could be causing poor flowering and fruiting?
2. Lack of Sunlight
Grapevines need full sun (at least 6+ hours of daily sunlight). This is how the plant gathers energy (sugar), leading to proper flower and fruit production.
Sunlight also helps fruit ripen and keeps fungus and mold from taking over and damaging the plant. While some grapevines can thrive off of less sunlight, it‘s not as common.
Here are some tips to boost sunlight for your grapevine:
- Plant the grapevine on the south side of your property for maximum sunlight.
- Prune overlapping and clustered branches to allow for more sunlight and airflow to reach more branches. This also helps prevents pests and diseases from developing and spreading. The best time to prune grapevines is in the late winter (more on pruning later).
3. Lack of Pollination
Grapevines without sufficient pollination develop few to no fruits or drop their fruits before ripening.
Flowers need to be pollinated (fertilized) to turn into fruit. If they’re not, the plant drops the unfertilized flowers and has minimal fruit yield.
Most grapevines are self-pollinating, but they benefit greatly from cross-pollination.
The commercial varieties of Vitis vinifera [Common Grapevine] are normally self-pollinated, but cross-pollination is possible. For many, if not most fruit crops, cross-pollination is essential for successful set.
The Australian Wine Research Institute
So, growing 2 or more grapevines of the same variety provides the best chance of successful pollination. While you can plant different grapevine varieties near each other, it can lead to hybrid fruits.
Another way to boost pollination is to plant grape companion plants nearby (within 25 to 50 feet). Many vineyards are discovering the benefits of companion planting and are starting to incorporate plants such as cover crops with their grapevines.
Additionally, pollination is best for grapevines between 79ºF to 90ºF. So, aim for these temperatures if possible.
4. Improper Nutrients
Excess Nutrients
Too many nutrients cause grapevines to have both poor flowering and fruiting.
And there are two main reasons behind this:
- Chemically burns the grapevine’s roots
- High nitrogen encourages foliage growth over flowering and fruiting
Chemical fertilizers with high amounts of nitrogen will stress the grapevine’s roots. This is especially true for fertilizers that are fast-releasing. This causes the grapevine to redirect its energy from reproducing (flowering and fruiting) to surviving the burns.
Additionally, high amounts of nitrogen encourage grapevines to focus on foliage growth (as nitrogen is the main nutrient used) instead of flowering and fruiting.
Generally, compost won’t cause either of these issues as it’s not potent enough.
Lack of Nutrients
While excess nutrients affect both flowering and fruiting, a lack of nutrients more commonly affects fruit production. This is because much less energy is needed to flower than fruit.
All plants need three main nutrients: nitrogen, phosphorus, and potassium (abbreviated as NPK).
To summarize the roles of NPK:
- Nitrogen is the primary nutrient, and the one responsible for foliage growth
- Phosphorus is a key nutrient in flowering and fruiting
- Potassium is vital to maintain the health and immune system of plants
While phosphorus is the main nutrient in flowering and fruiting, other nutrients such as potassium make it possible. So, avoid solely adding phosphorus to your grapevines.
Plants also need secondary nutrients such as magnesium, zinc, and iron.
Fortunately, most fertilizers and composts have sufficient primary and secondary nutrients. However, there are times when the grapevine’s soil could be lacking.
To help you diagnose the most common nutrient deficiencies with grapevines, check out this quick table I put together:
| Nutrient Deficiency | Leaf Symptom |
|---|---|
| Nitrogen | Entire leaf is pale or yellow |
| Iron | Dark green veins, rest of the leaf is yellowing |
| Zinc | Yellow blotches |
| Manganese | Broadly pale leaves, foliage color looks mottled or smeared |
The Best Fertilizer for Grapevines
Ideally, provide grapevines with an acidic fertilizer or compost.
However, many gardeners are finding that compost is replacing fertilizers for them. This is because chemical fertilizers commonly lead to long-term issues such as dry (dead) soil.
On the other hand, compost provides more than sufficient nutrients and promotes beneficial soil life (such as earthworms and mycorrhizal fungi). This soil life has benefits such as increased water retention, nutrient availability, disease resistance, and pest 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
Either option you choose, you can see my recommendations on my recommended fertilizer page. Since grapevines prefer a fairly acidic pH, using an acidic fertilizer works well.
Soil pH
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.
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
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. Over-Pruning
Over-pruning grapevines result in a lack of leaves necessary for photosynthesis. Because of this, the grapevine is unable to gather a proper amount of sugar to feed itself, causing little to no flowering and fruiting. If severely over-pruned, the grapevine can die.
However, when done right, pruning can dramatically assist with fruit production.
Pruning is not a necessary practice, but it’s helpful to:
- Encourage more fruiting
- Promote more sunlight and airflow
- Manage and prevent pests and diseases
- Shaping the plant into the desired appearance
Many grapevine growers prune their plant’s overlapping and excess branches to promote more and heavier fruiting. This works incredibly well as the plant has fewer branches and leaves to feed and can feed its flowers and fruits instead.
Ideally, prune grapevines in the late winter as pests and diseases are inactive and are less likely to infect the open wounds from pruning.
6. Pests and Diseases
| Common Pests for Grapevines | Common Diseases for Grapevines |
|---|---|
| Grape Phylloxera | Leaf Spot |
| Japanese Beetles | Tar Spot |
| Grape Berry Moths | Armillaria Root Rot |
| Mealybugs | Mildew |
| Hoppers | Verticillium Wilt |
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.
To see more about pests and how to treat grapevines, check out this resource by the University of Minnesota. You can also reference this resource by napagrowers.org.
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.
