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A striped watermelon growing among its vines
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When to plant melons in Ontario, OR

Plant melons in the garden in Ontario May 1–22, either as seeds or as young plants started indoors Apr 3–10.

Tricky here Plant seeds or young plants

Difficulty
Tricky here
Sun
Full sun; the hottest spot you have
Spacing
24 to 36 inches apart
Planting depth
1 inch deep
Frost
Killed by frost
First harvest
About 85 days after planting

Why these dates

Ontario's typical last spring frost is April 21. Frost kills melons, so planting starts once frost risk drops, around May 1. Keep row cover or a frost blanket handy for the first couple of weeks, since a late frost is still possible.

Columbia Basin and low eastern valleys. Hot, dry summers and quick spring warm-up. Warm crops go in earlier than west of the Cascades, and irrigation matters more than rain.

How to grow melons in Ontario

  1. Start seeds indoors Apr 3–10 in trays or small pots under a grow light or in your sunniest window. Or skip this step and buy young plants (starts) at a garden center.
  2. Harden off the young plants: about a week before planting, set them outside for a few hours a day, a little longer each day.
  3. Plant them in the garden May 1–22, 24 to 36 inches apart. If you plant seeds instead, sow them 1 inch deep.
  4. Water deeply once or twice a week rather than a little every day, and mulch once the soil is warm.
  5. Harvest: Cantaloupes are ripe when the stem slips off with a gentle push. Watermelons are ready when the curly tendril nearest the fruit turns brown and dry.

Key numbers

Growing notes

What makes the difference in Northwest gardens, in plain language.

Heat is everything

Melons come from hot climates, and their seeds rot rather than sprout in cold soil. Growth crawls whenever nights drop into the 50s, which is normal for June west of the Cascades. That's why black plastic mulch matters so much here: it warms the soil several degrees and keeps it warm overnight. Lay it a week or two before planting, then cover the plants with floating row cover.

Melon roots resent being disturbed, so if you start seeds indoors, use individual pots and transplant them young, before they get root-bound.

Flowers, bees and sweetness

Melons make male flowers first and female flowers, the ones with a tiny melon at the base, a week or so later. Bees have to carry pollen between them, so take the row cover off once female flowers open. Poorly pollinated fruit tends to be small or misshapen.

Sugar is made in the leaves, so healthy foliage means sweet fruit. Water deeply while vines grow and fruit swells, then ease off as melons near ripeness. Heavy watering at the end dilutes flavor and can split the fruit.

East vs west of the Cascades

Eastern valleys like the Yakima and Walla Walla areas have long, hot summers, and melons grow there with little fuss, including larger, longer-season varieties. West of the mountains, pick short-season varieties such as Sugar Baby, Minnesota Midget or Halona, and use every warming trick: the sunniest spot, black plastic, row cover, and a south-facing wall if you have one. Limit each vine to a few fruits so they ripen before cool September nights slow everything down.

Common questions

When is the last frost in Ontario, OR?

Typically April 21. In 9 years out of 10, frost is over by May 8. These are 1991–2020 NOAA climate normals for Ontario Municipal Airport.

Is it too late to plant melons in Ontario?

The spring planting window runs through May 22. Later plantings may not have time to produce before the first fall frost, which usually arrives around October 14.

When should I start melons from seed indoors in Ontario?

April 3 to April 10, which is 4 to 3 weeks before the May 1 planting date. Or buy young plants at a garden center instead.

Why aren't my homegrown melons sweet?

Usually the vines ran out of warm weather or healthy leaves before the fruit ripened. Picking too early, heavy watering near harvest and disease on the leaves also lower sugar. Short-season varieties, black plastic and fewer fruits per vine all help.

Can you grow watermelon in Portland or Seattle?

Yes, with small, early varieties like Sugar Baby and plenty of help from black plastic mulch and row cover. Expect a few modest fruits per plant rather than a big harvest, and expect better results in a hot summer than a cool one.

Why are my melon flowers falling off?

The first flowers are usually male, and they drop after a day or so by design. If female flowers (the ones with a tiny fruit behind them) also shrivel, they weren't pollinated. Remove row covers at bloom so bees can reach them.

Also plant in Ontario around the same time

Planting dates for melons in nearby areas

Full planting calendar for Ontario · Melons in every city