7 Reasons Your Zucchini Plant Isn’t Thriving This Year
Zucchini is usually an easy-to-grow crop and a prolific producer, but the cucurbits aren’t without their challenges in some growing conditions and areas. Gardening expert Katherine Rowe outlines common zucchini problems and remedies to get them thriving.
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Zucchini is a quick-developing crop that rewards us with high yields of glossy fruits all summer. The plants are versatile across garden scales, with compact, bushy types a fit for small spaces and containers. Easy to enjoy, the fruits move easily from the stem to the kitchen, fresh in summer meals, or frozen to toss in winter soups.
While normally an easy crop, challenges can arise during the growth of zucchini. Meeting a few key cultural requirements helps remedy common problems and build resistance to stave them off. Some, like squash vine borer, might be new to your growing area.
If your zucchini is not thriving, look to these common causes to try to revive the crop. Or, sow a successional round. It’s not too late to look forward to a late-season harvest, as bush types mature in as little as 55 days.
Planted Too Early
Heat-loving zucchini starts to take off in warm weather, growing quickly within a few weeks. Those long, sunny summer days and warm nights promote vigorous plants. If your vines are slow to start, it may be due to planting in cooler temperatures. Seeds need warm soils to germinate, and seedlings in chilly conditions may be slow to develop. The slowed growth hinders flowering and fruit set.
Direct sow seeds when the days and nights are warm. One to two weeks after the final frost, and when soil temperatures reach 75-85°F (24-29°C) is optimal for germination and growth. At a minimum, soil temperature should be consistently near 60°F (16°C) or above.
Poor Pollination
If your zucchini has flowers and produces odd, misshapen fruits, it may be due to weak pollination. They may show stunting or malformations, or may shrivel, dry, or drop prematurely. Zucchini, along with melons, cucumbers, and winter squashes, require frequent bee visits per bloom to set fruit. In areas or times when bee activity is low, so will be the number of fully formed fruits.
Zucchini produces male and female flowers that rely on pollen transfer from bees. The bees wriggle into the tubular blossoms for pollen and nectar. The large pollen granules stick to their fuzzy bodies and transfer from male to female blooms as they travel. Cucurbits need numerous repeat visits for adequate pollen transfer to the female flower.
Bee activity may be low in your area at a given time, especially during cool spells, rainy periods, if there isn’t much of a population, or if protective crop covers block access. Interplant with flowering herbs, perennials, and annuals to attract pollinators and other beneficial insects to the zone.
You can also try hand-pollinating, which is basically playing the role of the bee to transfer pollen from the male to female flowers. Pluck the male bloom and brush it across the female centers, or use a cotton swab or toothbrush to collect and distribute the granules between the two.
Bees are easier, and nectar-rich blooming plants draw more flying pollinators, who will, in turn, visit the zucchini blossoms.
Companion Plants
Companion plants, too, may boost a zucchini that’s not thriving by deterring pests and attracting beneficial ones. Good companions have natural essential oils that repel through scent, masking, or drawing beneficial predators that help manage pest populations.
Companions can also provide cooling shade, improve soils, and even enhance flavor. They share the same growing conditions as zucchini and don’t compete for resources.
Good zucchini companions include:
- Beans
- Corn
- Dill
- Marigolds
- Mint
- Nasturtium
- Tomato
- Yarrow
Water Fluctuations
Consistent moisture from regular, deep watering is best for zucchini. Usually, about one to two inches per week, from rainfall or irrigation, is sufficient. Check the soil moisture regularly, and water when the surface is dry one to two inches deep.
Fluctuations in water and imbalances from dry to wet situations cause stress, leading to halted growth. The crop weakens in adjusting to a lack of, or too much, moisture, and becomes more susceptible to pest and disease damage.
Take care not to overwater, as prolonged saturation is a top contributor to fungal diseases. Good drainage is essential for zucchini to thrive, and they prefer sandy, loamy soil. At planting, amend the soil with compost to improve nutrition, aeration, drainage, and moisture retention.
Blossom End Rot
Blossom end rot is a physiological disorder that likely results from water fluctuations. If you notice a rotting patch at the base of a ripening zucchini, it’s likely blossom end rot. In the same way it affects tomatoes, the fruit looks perfectly healthy until it suddenly begins to rot. Lengthy rainfall or hot, dry spells cause a lack of nutrient uptake, including calcium. Calcium is necessary to prevent BER.
At planting, scatter crushed eggshells or bone meal to promote calcium availability, and keep soil moisture consistent. The situation is reversible; amend with calcium supplements (those for tomatoes work well) to promote future healthy fruits.
Soil pH is another factor in calcium absorption. Soils with a low pH or high pH may inhibit availability. The cucurbits do best with a slightly acidic pH around 6.0 to 7.0. A soil test indicates pH and any recommended amendments, like garden lime or sulfur, to incorporate in the fall for the next growing season.
Low Airflow
Poor air circulation around leaves, stems, and fruits leads to zucchini not thriving. Low airflow promotes damp, humid conditions, coupled with warm temperatures that accelerate fungal problems.
Planting the cucurbits in low hills can improve drainage and airflow. Sow two to three seeds in a raised mound. When three sets of true leaves appear, thin the seedlings so that one strong, singular sprout remains. Clip them at the base of the stem to avoid disturbing the young roots of the remaining sprout. Add straw or mulch around the crowns to retain moisture and regulate soil temperatures.
Zucchini grows beautifully in pots and raised beds, which can improve drainage and airflow over poor soils. Grow them vertically to boost air circulation by lifting the stems off the ground. Training them upright on vertical supports also saves valuable growing space.
Diseases
The cucurbits are susceptible to a number of common diseases. If you notice distinct yellow and brown spots, you may be dealing with anthracnose, a common leaf blight. Fuzzy, white spots or a coating on leaves indicate powdery mildew, and with yellowing speckling, downy mildew.
A horticultural oil, like neem, limits the spread of the disease to healthy plant tissue. Ample air circulation and avoiding splashing the leaves through irrigation help stave off the spread. Water at the ground level through drip irrigation or soaker hoses to avoid overly wet conditions (spores spread when they splash from the soil onto the leaves). Mulch around the crop to avoid splashing water while retaining moisture and regulating soil temperatures.
Disinfect tools after pruning off diseased leaves before moving to another plant. An alcohol wipe, a dip, or a spray with 70% isopropyl alcohol does the job.
Overfertilizing
If your zucchini lacks flowers and fruits, first check the sun exposure and temperature. Full sun, with six or more hours, is optimal. If sunlight and warm temperatures are in place, overfertilizing may be the issue.
The cucurbits are heavy producers and benefit from added nutrients to boost vigor, blooming, and fruiting. In nutrient-rich soils, they won’t need much. At planting, topdress with a balanced organic granular fertilizer. As the crops approach flowering, switch to one higher in phosphorus to support blooming and fruit set. A tomato formula works well for squashes. Fish emulsion at planting and seaweed and bone meal during flowering and fruiting are mild organic options.
Avoid overfertilizing with too much nitrogen. Nitrogen produces leafy vines but reduces flowering, as the plant directs energy into foliage and stem growth rather than fruit production. Skip applying excess nitrogen through manure and compost during the growing season.
Sudden Wilt
If leaves or vines are wilting, the first thing to check is soil moisture. If the soil feels consistently moist (not overly saturated and not dry), then the wilt has an underlying cause.
Fusarium and Verticillium wilt are fungal diseases that cause wilting, blackened leaves and stems, and quick decline. Wilts spread from spores in the soil and can come with infected seeds or transplants. Wind, water, and garden tools also spread the spores. The fungus impacts the roots and limits nutrient and water uptake. With fungal wilts, you may see one half of the plant wilting more than the other.
Unfortunately, there’s no treatment for wilt. Remove and dispose of affected plants, keeping them out of the compost pile. Disease-resistant varieties can limit the spread. If you notice squash bugs on your vine, remove them as they are a vector for bacterial wilts.
Squash Vine Borer
Another cause of sudden wilting may be due to squash vine borers, a pest that is becoming more widespread. The insects occur east of the Rocky Mountains and from Canada to South America. They’re not yet a common problem in the West, but occur in the Midwest, along the East Coast, and particularly in the Southern U.S.
The adult female moths lay a single egg along the stems, usually near the base. Larvae hatch and burrow into the stem to feed on plant tissues. Unable to absorb water, the vine suddenly wilts.
There are ways to lessen the borer’s damage, the best being regular scouting and growing zucchini away from other cucurbits in areas where the pest is active. If you live in an SVB-prone area, a preventative measure is to opt for a zucchini stand-in. Summer squashes are one of the most impacted crops. Try resistant Cucurbita moschata varieties like ‘Tromboncino.’ While technically a winter squash, it makes a delicious zucchini substitute when harvested young and green.
Other Significant Pests
As members of the Cucurbitaceae family, summer squash is susceptible to the same pests as melons, cucumbers, and winter squashes. In addition to the squash vine borer, the cucumber beetle and the squash bug are the most likely insects to damage the crop.
The squash bug is a sap-sucking pest that results in the eaves losing water and nutrients. The leaves become speckled, and eventually wilt, blacken, and drop.
Cucumber beetles are chewing insects that consume all parts of the zucchini, from stem to leaf to fruit. You may notice their damage as irregular, jagged holes in the leaves. Adults feed on leaves, blossoms, and fruits, while larvae feed on roots and stems. They’re also vectors for bacterial wilt.
Regular scouting for each pest benefits the crop. Floating row covers help protect seedlings as they establish by blocking the pests from feeding on tender growth. Remove the covers as the young plants mature and begin to flower to ensure pollination.
Pyrethrin sprays, spinosad, and neem oil treat the cucumber beetle. They’re most effective with regular treatments for transplants and seedlings during times of high beetle activity, though they impact pollinators and other beneficial insects as well. However, hand picking them is more effective.
