When to Plant Zucchini in Ontario — 2026 Guide

1 plant

Produces 6–10 zucchinis per week at peak. Enough for a couple. You will still be giving them away.

2 plants

Maximum for a family of 4. You will definitely be leaving bags of zucchini on neighbours' porches.

3+ plants

Unless you're growing for a market or large group, this is too many. You will be overrun by August.

The standard Ontario zucchini. Dark green, reliable, widely available. Bush habit — doesn't sprawl too far. Works across all Ontario zones. The benchmark.

Fastest and most compact. Excellent for containers and small raised beds. Very productive for its size. Best choice for Ottawa and Kingston where season length matters.

Yellow zucchini, same growth and harvest as Black Beauty. Great visual variety. Easier to spot in the garden — less likely to miss hidden ones that become marrows.

Round zucchini, tennis ball to baseball size at harvest. Compact plant. Excellent for stuffing. Harder to miss at harvest — no hiding under leaves. Works all Ontario zones.

Italian ribbed zucchini, exceptional flavour compared to standard varieties. Larger plant — needs space. Best for Toronto and Windsor where the longer season rewards this slightly slower variety.

Bred specifically for containers. Compact, upright plant, produces full-size zucchinis in a 40-litre pot. Best container zucchini for Ontario balconies and patios.

Wait for 18°C soil — speed gains nothing in cold ground

Zucchini germination fails in cold soil below 15°C — germination rates drop and grows slowly — often not catching up to seeds sown 2 weeks later in warm soil. In Ontario, soil is often still cool in mid-May even after last frost. Use a soil thermometer to confirm 18°C before sowing. Black plastic mulch laid a week before sowing warms soil by 3–5°C. A warm June sowing consistently outperforms an anxious May sowing in cold ground.

Harvest at 15–20 cm — check every 2 days

This is where most Ontario gardeners go wrong. A zucchini left on the plant grows from ideal harvest size to marrow in 3–4 days in Ontario's summer heat. Check plants every 2 days at peak season. Harvest at 15–20 cm for best flavour and texture, and to keep the plant producing. A zucchini that grows to marrow size tells the plant its reproductive job is done — it slows production dramatically. Yellow-skinned Gold Rush and round Eight Ball are easier to monitor because they stand out visually.

Flowers but no fruit — understand pollination

Zucchini produces male flowers for 1–2 weeks before the first female flowers appear — this is normal and not a problem. Male flowers have a thin straight stem; female flowers have a tiny zucchini at the base. Once both types are open simultaneously, bees do the work. If you're seeing female flowers with small zucchinis that rot without growing, hand-pollinate: use a small brush or cotton swab to transfer pollen from a male flower to the centre of a female flower. Do this in the morning when flowers are fully open.

Space generously — zucchini gets enormous

One zucchini plant needs a minimum 90 cm diameter growing space — ideally 120 cm. In a square foot garden, allocate 4–6 squares to one plant. Cramped zucchini has poor air circulation, leading to powdery mildew, and the leaves shade out everything nearby. Plant at the north end of a bed so the sprawling growth doesn't shade shorter vegetables. Bush varieties like Patio Star and Eight Ball are significantly more compact and better suited to smaller Ontario gardens.

Blossom end rot — consistent watering is the fix

Small zucchinis that turn dark and rot at the blossom end before reaching full size are showing blossom end rot — the same condition that affects tomatoes and peppers. It's caused by calcium deficiency from inconsistent watering rather than lack of calcium in the soil. The calcium is in the soil but inconsistent moisture prevents uptake. Water consistently and deeply, keeping soil evenly moist. Mulching around plants significantly reduces the feast-famine moisture cycles that cause blossom end rot in Ontario's summer heat.

When should I plant zucchini in Ontario?

Direct sow after last frost when soil is above 18°C. Toronto and Windsor: May 20 to June 1. Ottawa and Kingston: June 1 to 10. Never start indoors for transplanting — zucchini grows faster from direct sowing in warm soil. Use the frost calculator for your city's exact last frost date.

How many zucchini plants should I grow?

One to two plants maximum for a household. One plant produces 6–10 zucchinis per week at peak. Two plants will have you leaving bags on neighbours' porches. Three or more and you'll be overrun by August. Plant 2 seeds per spot, thin to 1 plant when established, and plant no more than 2 spots.

Why do my zucchini flowers fall off without producing fruit?

Early in the season, zucchini produces male flowers only — these always drop off, producing no fruit. Female flowers (with a tiny zucchini at the base) appear 1–2 weeks later. Once both types are open simultaneously, pollination occurs and fruit develops. If female flowers appear but rot without growing, hand-pollinate by transferring pollen from a male to female flower with a small brush in the morning.

What size should I harvest zucchini?

Harvest at 15–20 cm (6–8 inches) for best flavour and to keep the plant producing. Zucchinis left to grow larger become seedy, watery, and tough-skinned — this is what becomes a marrow. Check plants every 2 days in peak season. In Ontario's summer heat, a zucchini can go from ideal to oversize in 3–4 days.