Planting calendars

Mountain-town planting calendar

A month-by-month calendar for high Colorado valleys, from Alamosa and Salida to Gunnison, Steamboat, Dillon, and Crested Butte.

By the Thin Air Harvest editorial team. Updated November 1, 2026.

Above about 7,500 feet, Colorado State University Extension’s mountain fact sheet stops treating the garden as a shorter Front Range and starts treating it as a different climate. Many of those gardens have fewer than 90 frost-free days.

Nights stay cool. The crops that pay are greens, roots, peas, brassicas, and potatoes. Tomatoes are a covered experiment. This calendar is the month-by-month version of that judgment, tied to stations instead of to a slogan.

The spring dates are medians and 1-in-10 late dates for a 32°F threshold from NOAA’s Colorado freeze-occurrence table. Read them with the method in how to read a spring frost. Gilpin County’s figures are a separate CSU source and are labeled that way.

Gilpin County Extension, for a community garden at 9,300 feet, publishes an average last frost of June 10 and an average first frost of September 15, and suggests adding about 20 days to packet maturity. That June 10 is earlier than Dillon’s July 7 median.

Different instruments, different products. If you garden near either one, quote the one that matches your elevation and your valley, and keep a thermometer at plant height. Do not average them into a date nobody measured.

Climax, Dillon, and Crested Butte are alpine and high-valley stations. A median last freeze in July means June is still spring. Plan the garden as a cool-season garden that starts in earnest after snow leaves the soil, and stop expecting a Front Range July.

March and April

These are planning and indoor-seed months, not outdoor planting months, except in the mildest mountain sites during a mild year. Start brassicas and onions indoors on the schedule in days to maturity, counting back from a set-out date about two weeks before your median, not from a Front Range April.

In a Dillon year the median is July 7, so a brassica started in March would be overgrown and stale long before the soil is ready. Start later.

Sharpen tools. Order short-season seed. Do not be talked into tomato six-packs at a valley nursery on a warm April Saturday if your soil is still frozen.

May

Snow may still be the surface. Where soil has drained and crumbles, you can work compost into beds and set up cold frames. Outdoor sowing of hardy crops waits until about four weeks before your median, or until the soil at 6 inches is about 40°F at 8 a.m., which is the mountain sheet’s rule.

For Salida or Alamosa, with medians near June 7 to June 9, that window opens in May. For Gunnison and Steamboat, with medians of June 27, May is still early for open-ground sowing. For Dillon and Crested Butte, May is not planting season in the open.

Harden anything you started indoors. Mountain sun burns leaves that only saw a grow light. Wind finishes the job if you skip the hardening.

June

This is spring. Around Alamosa, Salida, and Buena Vista, sow peas, spinach, lettuce, radish, and other hardy crops as the soil allows, and set out hardened brassicas. Potatoes go in two to three weeks before the median, pre-sprouted.

A June 9 median means potato planting in the second half of May or the very start of June, not in April. In Gunnison and Steamboat, the hardy sowings land in June.

In Dillon, a median of July 7 means the four-week-before window is June, and only if the snow is gone.

If it is not, wait. A date does not thaw a bed.

Do not set tomatoes out in the open. A short-season determinate under a cover, in the warmest corner, is the only tomato plan, and it is optional. The warm-season page explains why the arithmetic usually fails.

July

In the lower high valleys, this is the growing month. Keep lettuce sowed in small amounts.

Hill potatoes. Cover brassicas against flea beetles if you did not cover them at sowing. In Crested Butte and at Climax, July is when the last freeze median finally arrives or has just arrived.

Planting is still happening. Growth will be fast once nights are merely cool instead of freezing, and it will not last. Favor crops you can eat as leaves and roots.

A bean sowing on July 16, the Gunnison late date, can still finish a 50-day bush bean if frost holds off into September. It will not finish a 100-day squash. Do that math before you spend the bed.

Water. Mountain air is dry, and a bed that thawed late is not a bed with stored moisture. Drip and a mulch that will not blow away are the system.

Hail happens in the mountains too. A simple frame over the salad bed is worth more than a third tomato.

August

Harvest what is ready. Sow only if your first frost is documented late enough to mature the crop.

Gilpin’s September 15 average first frost means an August sowing of radish or spinach can work there, and a new planting of winter squash cannot.

High valleys with a July last frost often see frost again in late August or early September.

Watch the forecast. Have covers for the salad bed. Do not side-dress nitrogen late. Soft growth will not harden.

September

Frost is the expected event, not the surprise. Harvest potatoes after the vines die, or before a hard freeze ruins the tops and you are guessing.

Pull green tomatoes if you grew them and a freeze is forecast. They may not be worth the fuss. Mulch carrots if you want to keep lifting them. Clean irrigation so a freeze does not split the lines.

Write down the first 32°F morning. Compare it with Gilpin’s September 15 or with whatever fall figure you actually have. If you do not have a fall figure, do not invent one from the spring median.

October through February

Plant garlic only if the soil is still workable and you can mulch after it freezes.

In many high towns the ground is done earlier than on the Front Range, so garlic goes in during September. Drain water systems. Protect trunks from voles under the snow.

Order seed for the crops that worked, and drop the ones that were theater. Review whether the tomato earned its square footage. Most years the honest answer is no, and the peas did.

Checklist

• Treat June as spring unless your own station’s median is earlier.
• Use the four-week rule and a soil thermometer, not a Front Range weekend.
• Grow hardy crops and potatoes as the main garden.
• Add about 20 days to packet maturity near 9,300 feet, following Gilpin’s summary, until your notes say otherwise.
• Skip long-season warm crops. Cover a short-season tomato only if you can spare the space.
• Record the first fall frost. Do not guess it.
• If your town is not in the table, use the Climate Center tool or the nearest similar elevation and say so in your notebook.

The Front Range calendar will mislead you if you use it here. The Utah and Idaho pages have their own high-valley stations, which behave more like this page than like Salt Lake or Boise.

Sources

• CSU Extension, Vegetable Gardening in the Mountains.
• Gilpin County Extension climate summary for the 9,300-foot garden.
• NOAA NCEI, Colorado freeze/frost occurrence data.
• Colorado Climate Center, 1991–2020 freeze tool.