Rainline Garden My garden plan
Coral-pink zinnias blooming in a garden bed
Photo: Pexels

When to plant zinnias in Hermiston, OR

Plant zinnias from seed, directly in the garden in Hermiston, May 3 – Jun 14.

Easy Plant seeds in the ground

Difficulty
Easy
Sun
Full sun
Spacing
9 to 12 inches apart
Planting depth
¼ inch deep
Frost
Killed by frost
Flowers
Blooms about 60 to 75 days after sowing

Why these dates

Hermiston's typical last spring frost is April 26. Frost kills zinnias, so planting starts once frost risk drops, around May 3. 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 zinnias in Hermiston

  1. Prepare the bed: loosen the soil and mix in an inch or two of compost.
  2. Plant seeds May 3 – Jun 14, ¼ inch deep, 9 to 12 inches apart.
  3. Keep the soil evenly moist until sprouts appear, which can take one to three weeks in cool soil.
  4. Harvest: Cut when stems are stiff right below the flower (give it a gentle wiggle test). Cut long stems often; the more you cut, the more they bloom.

Key numbers

Growing notes

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

Wait for warm weather

Zinnias come from hot, dry regions and do nothing in cold soil. Seeds sown too early west of the Cascades often rot, and seedlings sit stunted and yellow. Sow once the soil is warm and nights are mild, usually late May to early June on the west side. They catch up fast: a sowing in early June often blooms by midsummer.

You can start them indoors a few weeks early, but transplant them young, since older seedlings don't like root disturbance.

Pinching and cutting

When a young plant is about a foot tall with three or four pairs of leaves, pinch out the top just above a leaf pair. The plant branches, and those branches give more stems and longer ones for cutting.

Cut flowers often and cut deep, down to a leaf pair low on the stem, so new branches have room to grow. Check for ripeness with the wiggle test: a stem that stays stiff just below the flower will last in the vase, while a floppy one wilts quickly.

Mildew, and east vs west

Powdery mildew coats leaves with white powder in late summer, when warm days and cool, damp nights alternate, a typical west-side pattern. Space plants for airflow, water the soil instead of the leaves and choose resistant varieties like Zahara. It looks bad but rarely stops flowering.

East of the Cascades, zinnias thrive in the heat and mildew is less of a problem, though steady water keeps them blooming. Slugs on young seedlings are the bigger west-side issue.

Common questions

When is the last frost in Hermiston, OR?

Typically April 26. In 9 years out of 10, frost is over by May 10. These are 1991–2020 NOAA climate normals for Hermiston Municipal Airport.

Is it too late to plant zinnias in Hermiston?

The spring planting window runs through June 14. Later plantings may not have time to bloom well before the first fall frost, which usually arrives around October 10.

When do zinnias bloom?

Blooms about 60 to 75 days after sowing. Cut when stems are stiff right below the flower (give it a gentle wiggle test). Cut long stems often; the more you cut, the more they bloom.

Should I pinch my zinnias?

Yes, for cut flowers. Pinch the top when plants are about a foot tall; they'll branch and give many more long stems. Skip pinching on dwarf bedding types.

What is the white powder on my zinnia leaves?

Powdery mildew, a fungus common in late summer. Remove the worst leaves, water at the base and give plants more room. Next year, try resistant varieties like Zahara.

Why don't my zinnias last in a vase?

They were likely cut too early. Use the wiggle test, cut in the cool of the morning, strip the lower leaves and change the water often. Zinnias prefer clean water without much floral preservative.

Also plant in Hermiston around the same time

Planting dates for zinnias in nearby areas

Full planting calendar for Hermiston · Zinnias in every city