Planting Guides
When to Plant Onions in Philadelphia: Complete Guide + Best Varieties for Zone 7a
Philadelphia, Pennsylvania
USDA Zone 7a
Last Frost: Apr 10
Last updated: October 30, 2025
Plant onions in Philadelphia March 10-25 for spring harvest or September 15-October 1 for early summer harvest. Zone 7a supports intermediate-day varieties.
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Quick Answer
Plant onions in Philadelphia March 10-25 for summer harvest or September 15-October 1 for overwinter crop. Choose intermediate-day varieties for Philadelphia's latitude.
TL;DR
Philadelphia gardeners in Zone 7a should plant onions between March 10-25 for summer harvest or September 15-October 1 for early summer harvest. Choose intermediate-day varieties like Allium cepa 'Candy' and Allium cepa 'Red Candy Apple' that perform well at Philadelphia's 40°N latitude. Plant garlic cloves in October 15-November 15 for next summer's harvest.
Frequently Asked Questions
When is the latest I can plant onions in Philadelphia?
March 30 is the absolute latest for spring planting in Philadelphia. Onions planted later may not have enough growing time before day-length triggers bulbing. For best results, plant between March 10-25 when soil temperatures reach 35°F consistently. Fall planting offers another window from September 15-October 1 for overwintering varieties.
What happens if I plant short-day onions in Philadelphia?
Short-day onions will likely fail to bulb in Philadelphia because they need only 10-12 hours of daylight to trigger bulbing. Philadelphia's summer days reach 14-15 hours, so these varieties stay in vegetative growth producing large tops but small or no bulbs. Choose intermediate-day varieties (12-14 hours) for reliable success at Philadelphia's 40°N latitude.
Can I grow onions from seed in Philadelphia's climate?
Yes, Philadelphia's 209-day growing season is long enough for seed-grown onions. Direct sow seeds March 15-30 when soil temperature reaches 40°F consistently. Seeds take 4-6 weeks longer than sets but offer better variety selection and no bolting risk. Start indoors in February for even earlier transplants.
How deep should I plant garlic cloves in Philadelphia?
Plant garlic cloves 2 inches deep, pointed end up, in Philadelphia. This depth provides adequate winter protection in Zone 7a while allowing spring emergence. Space cloves 6 inches apart in rows 12 inches apart. Plant October 15-November 15, about 4-6 weeks before ground freezes hard, allowing root establishment before winter dormancy.
Why do my onions have thick necks that won't cure?
Thick necks usually result from excess nitrogen late in the growing season or too much moisture near harvest. Stop fertilizing by mid-June in Philadelphia and reduce watering as tops begin yellowing. Harvest only when 50-75% of tops have fallen over naturally, and cure in warm, dry conditions with good air circulation.
What's the difference between hardneck and softneck garlic for Philadelphia?
Both types grow well in Philadelphia's Zone 7a climate. Hardneck varieties like Allium sativum 'Music' produce scapes (flower stalks) and typically have fewer but larger cloves with complex flavor. Softneck varieties like Allium sativum 'California Early' don't produce scapes, have more cloves per bulb, and store longer. Choose based on your cooking preferences and storage needs.
Can I plant onions and garlic in the same bed in Philadelphia?
Yes, onions and garlic make excellent companion plants and can share garden space in Philadelphia. Both prefer similar soil conditions—well-draining, fertile soil with pH 6.0-6.8. Plant garlic in fall (October-November) and onions in spring (March) or fall (September) depending on variety. Space appropriately: garlic 6 inches apart, onions 4 inches apart.
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Frank Russo
Frank retired from the city parks department after 28 years, the last fifteen as a grounds supervisor overseeing park plantings and maintenance crews. He now spends his time volunteering at a local historic garden, maintaining his own half-acre property in the Roxborough neighborhood, and writing about landscape maintenance, seasonal timing, and the practical side of keeping plants healthy through Mid-Atlantic winters and humid summers. Frank knows what works at scale and what's a waste of time—he's planted thousands of trees and shrubs and watched plenty of them fail. His advice tends toward what's proven and durable rather than what's trendy.
