Planting calendars

Front Range planting calendar

Month-by-month vegetable work for Denver, Boulder, Fort Collins, Colorado Springs, and the colder Palmer Divide, using NOAA station medians.

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

The Front Range is not one frost date. Denver’s old Stapleton station and a town on the Palmer Divide are weeks apart in the same NOAA table. This calendar uses that table’s last spring 32°F dates and the crop classes in Colorado State University’s planting guide.

The way to read a median versus a 1-in-10 late date is in how to read a spring frost. If your garden sits in a hollow that frosts later than the airport, believe the hollow.

Elevations below are approximate city elevations, not the exact station height. The dates are the station’s.

Grand Junction’s Walker Field median, April 25, with a late date of May 9, is in the same table and is not the Front Range. It is what a lower, warmer western valley looks like. Do not use it in Castle Rock.

Castle Rock’s median of May 22 is the number that surprises people who learned to garden in central Denver. Shift every task below by about three weeks if Castle Rock, or a similar cold bench, is your station. Pueblo can run a few days earlier than Denver on some crops and still gets the same hail and wind.

The Colorado Vegetable Guide suggests shifting its own baseline advice by about one day per 100 feet above or below 6,300 feet. Use that only to nudge a recommendation. The table above is better where your town is actually listed.

Confirm a newer normal at the Colorado Climate Center if you are planting at commercial scale. The file these medians come from is in NCEI’s deprecated-normals archive. It is still the clearest side-by-side public comparison.

March

Soil is usually too wet to dig on clay, and nights are still winter.

Order seed. Start onions indoors about ten weeks before your median last frost, which for a Denver median of April 30 means a late-February or early-March sowing. The vegetable guide is the source of that ten-week onion lead.

Start broccoli and cabbage indoors later in the month if you want transplants sized for mid-April.

Clean beds. Do not till wet clay. Prune fruit on a dry day. The clay page is the ribbon test that tells you to wait.

April

This is the hardy-crop month around Denver, Boulder, Fort Collins, and Colorado Springs, and it is still early on the Palmer Divide. As soon as the soil crumbles, sow peas, spinach, lettuce, radish, and the other hardy crops CSU lists, often two to four weeks before the median last frost.

In a Denver yard that can be early April. In Castle Rock, treat April as March and wait until the soil and the later median agree. Set out hardened brassica transplants around the middle of the month only in the warmer Front Range cities, not on a cold bench.

A warm week is not the end of frost. Denver’s late date is May 13. Keep row cover ready.

Potatoes can go in two to three weeks before the median, pre-sprouted, which for Denver is mid-April and for Castle Rock is early May. Do not set tomatoes out because the garden center is busy.

May

May is the transition. Around the median, sow beans, corn, and summer squash if days are staying above about 55°F and you can cover a surprise night.

Wait until after the late date for tomatoes, peppers, and eggplant unless they live in a cold frame you will vent. On the Palmer Divide that late date is June 11. Planting tomatoes in Castle Rock on Mother’s Day is a Denver habit in the wrong town.

Hail season, as CSU’s CO-Horts blog cites it from the Colorado Climate Center, is already underway. Have the hail cover built or staged.

Thin the April sowings. Mulch hardy crops after the soil has warmed. Start a second small sowing of lettuce and radish.

June

Warm-season crops should be in the ground in Denver, Fort Collins, Boulder, and Colorado Springs by early June, even in a slow year.

On the Palmer Divide, early June is still planting time for tomatoes. Sow a second bush bean if the first is up. Stop planting peas on the plains once heat is settling in. Keep watering.

Wind and long days will dry a raised bed faster than the calendar suggests. Side-dress brassicas if they pale. Set the hail cloth and leave it.

July

Harvest, do not redesign. Plant a fall crop of broccoli, cabbage, or carrots only if you are in a city whose autumn is long enough.

The vegetable guide’s mid-July fall cabbage sowing is a baseline for mid-elevation Colorado, not for a cold bench that may freeze in September. Shade lettuce or sow a bolt-resistant type.

Keep tomato water even so fruit does not split. Watch for hail and for the first spider mites on beans in a hot, dry spell. Water deeply in the morning.

August

Sow fall spinach and lettuce in the warmer Front Range cities, and a last radish.

Stop sowing corn and squash. Harvest garlic if the lower leaves have yellowed and you planted it the previous fall. Start onions curing if the tops have fallen.

Cover ripening tomatoes only if an odd early cold is forecast. Most years the risk is heat and hail, not frost. Renew mulch that the wind has thinned.

September

This is the month the Colorado Climate Center’s fall-freeze writing tells you not to get casual about. Early freezes happen.

Keep a cover within reach of the tomatoes and peppers. Harvest ripe fruit before a cold night, and pick mature green tomatoes if a harder freeze is forecast.

Fall lettuce and spinach should be up. Peas are usually finished. A row cover over the salad bed extends the month. Do not fertilize woody plants.

October

First frosts become likely in many Front Range yards. Clean up diseased tomato vines. Leave healthy roots in the ground under mulch if you want to keep harvesting.

Plant garlic as the soil cools, then mulch it after the surface starts to freeze, not before. Drain drip lines.

Plant hardy perennials and trees so roots grow in cool soil, and water them. Rake hail cloth off if the season is over, and store it before the wind does.

November through February

Water trees and evergreens on mild days when the soil is dry. Check deer and rabbit fencing before the hungry months. Order seed.

Do not prune during a bitter cold snap. Review the notebook: the date you actually hit 32°F in spring and fall, at plant height. That record will eventually beat Stapleton.

Checklist

• Use your own row in the table. Castle Rock is not Denver.
• Shift Palmer Divide tasks later by the gap between your median and Denver’s April 30.
• Sow hardy crops when soil can be worked, often in April around Denver.
• Hold tomatoes until after your late date, or cover them.
• Keep hail cover ready from mid-April through mid-September.
• Plant garlic in fall. Mulch after freeze-up.
• Write down the frosts you actually see.

Crop details are in the vegetable section. If you are above about 7,500 feet, leave this page for the mountain-town calendar.

Sources

• NOAA NCEI, Colorado freeze/frost occurrence data.
• Colorado Climate Center, freeze normals tool and first fall freezes.
• CSU Extension, Vegetable Planting Guide and the Colorado Vegetable Guide.
• CSU CO-Horts, hail season citing the Colorado Climate Center.