Frost Dates for Helena, MT 59623

Last spring frost June 6 · First fall frost September 12 · 98 frost-free days · USDA Zone 5a

Frost probability for 59623

Frost10%50%90%
Spring (last frost)June 21June 6May 21
Fall (first frost)August 31September 12September 23

Dates come from NOAA 1971–2000 freeze probabilities at the nearest reporting station (18.3 km away); the 50% date is the median year.

Month by month in Helena

MonthWhat to do
JanuaryPlan your garden layout and order seeds. Research varieties suited to your frost dates.
FebruaryStart the slowest-growing crops indoors: onions, leeks, and celery (10–12 weeks before last frost).
MarchOrganize planting supplies and seed-starting mix. Check stored garden tools.
AprilStart peppers and eggplant indoors 8–10 weeks before your last frost (around June 6). Start tomatoes about two weeks later, 6–8 weeks before last frost.
MayStart herbs, basil, and squash indoors (4–6 weeks before frost). Direct sow peas, spinach, and lettuce if soil is workable.
JuneLast frost around June 6. Transplant cold-hardy crops (broccoli, kale, cabbage). Harden off tomato and pepper seedlings.
JulyFrost has passed. Transplant tomatoes, peppers, and basil outdoors. Direct sow beans, cucumbers, squash, and zucchini.
AugustFirst fall frost approaching (around September 12). Transplant fall brassicas. Sow spinach, arugula, and radishes.
SeptemberFirst frost around September 12. Harvest tomatoes and peppers before frost. Cover cold-hardy crops with row covers to extend the season.
OctoberHarvest remaining cold-hardy crops: kale, carrots, and beets. Plant garlic bulbs for next year.
NovemberClean up spent plants. Add compost to beds. Rest, review, and plan next year's garden.
DecemberClean up spent plants. Add compost to beds. Rest, review, and plan next year's garden.

What grows in a 98-day season

Frequently Asked Questions

What is the last frost date for Helena, MT (ZIP 59623)?

The median last spring frost date for Helena, MT (ZIP code 59623) is around June 6. This is the 50th-percentile date — meaning half of all years see frost end before this date. Use the 10% and 90% dates to understand the full range of risk in your area.

What is the first fall frost date in Helena, MT?

The median first fall frost date for Helena, MT is around September 12. Plan to harvest or protect frost-sensitive crops like tomatoes and peppers at least a week before this date to avoid losses.

How long is the growing season in Helena, MT?

The frost-free growing season in Helena is about 98 days. This window — from your last spring frost to your first fall frost — is when warm-season vegetables thrive. Cold-hardy crops like kale, spinach, and peas can extend your season by weeks on either side.

What USDA hardiness zone is Helena, MT?

Based on frost date patterns, Helena falls approximately in 5a. USDA hardiness zones are determined by average annual minimum winter temperatures and guide both plant selection and planting timing for your location.

When should I start tomatoes indoors in Helena, MT?

Start tomato seeds indoors 6–8 weeks before your last frost date (around June 6). For Helena, that's roughly April 11 to April 25. Transplant outdoors 1–2 weeks after your last frost.

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