Cool-season crops that suit the high West
Peas, brassicas, lettuce, spinach, and roots use the cool nights other regions fight. Timing, pests, and a fall sowing on the Front Range.
The high West is a cool-season climate that happens to have hot, bright afternoons. That combination is awkward for tomatoes and generous to peas, brassicas, lettuce, spinach, and roots. Colorado State University’s planting guide puts broccoli, cabbage, kohlrabi, onions, lettuce, peas, radish, spinach, and turnips in the hardy group: they grow with daytime temperatures down around 40°F and they survive a light frost.
The mountain vegetable sheet goes further. Direct-sow the frost-tolerant list about four weeks before the local last frost, or when a soil thermometer reads about 40°F six inches down at 8 a.m. The list there adds kale, chard, mustard, beets, carrots, endive, and the brassicas and roots already named.
If the soil is frozen or the bed is still under snow, the four-week rule waits. A date on a card does not thaw a bed.
This is the crop group to master before you spend money on a tomato cage. The maturity math is in days to maturity. The station dates are in the calendars.
Peas want the cool and hate the July heat
Sow peas as soon as the soil can be worked, in the four-week window before the median last 32°F date. On the Front Range that often means late March or April, and it means June in a town whose median falls in late June.
Inoculant is cheap insurance if you have not grown a legume in that bed. A low trellis is worth building before the vines need it. Wind will flatten an unsupported row, which is a wind problem as much as a pea problem.
The Front Range failure is heat, not frost. Once days sit in the 80s and nights stop cooling into the 50s, bloom drops and the vines tan.
A spring crop that was sown early is the one you eat. A second sowing in midsummer for fall only works where September is long enough to flower and fill pods before a hard freeze. That is a Front Range and valley-Utah project.
It is a poor bet in Gunnison or at Dillon, where the NOAA median last spring freeze is already June 27 or July 7 and the autumn is not a second spring. In the mountains, peas can keep producing through a mild summer because the nights never get hot. That is the consolation for a short season.
Choose shorter, disease-tolerant cultivars if you have had powdery mildew. The species is the point. A pea that needs 75 days still fits a Front Range spring.
A pea that needs 75 days and a hot July does not exist. They are a cool crop.
Brassicas: cover them the day you sow
Broccoli, cabbage, kale, kohlrabi, and cauliflower are the dependable headed and leafy crops above 6,000 feet, with one insect catch. Flea beetles and imported cabbageworm will find an uncovered seedbed. A lightweight row cover pinned at the edges, from the day of sowing or transplanting, prevents both better than a spray program you will start too late.
The Colorado Vegetable Guide notes that covers can hold air and soil up to about 10°F warmer than the open garden. On a brassica bed that warmth is a bonus. The insect exclusion is the reason.
CSU’s mountain sheet says broccoli, cauliflower, leeks, and Brussels sprouts can be started indoors 6 to 8 weeks before the last frost and set out about two weeks before it. Harden them.
A soft transplant in a 40°F wind stalls. Space them for the head you want. The vegetable guide notes that crowded cabbage makes small heads, and that transplants around five to six weeks old handle bad weather better than older, pot-bound ones.
Its baseline timing, about April 15 for spring transplants and a mid-July sowing for fall cabbage, is written for a mid-elevation Colorado season. Shift it with your station. At Alamosa, median last freeze June 9, a mid-April transplant is a month early.
Fall cabbage and broccoli are realistic on the Front Range, in the Wasatch valleys, and in other places with a September that stays open. Start the fall crop in midsummer so it matures into cool weather, which is when the flavor is best. Above 8,500 feet, one spring planting that heads in August is the crop. A second planting competes with the first for the only warm month you have.
Cauliflower is the prima donna of the group. It buttons if it is stressed young by cold, crowding, or a dry week. If your season is marginal, grow broccoli and kale, which forgive a check in growth, and treat cauliflower as optional.
Lettuce and spinach are a timing crop
Head lettuce is one of the vegetables the Colorado Vegetable Guide specifically calls a good mountain crop.
Leaves are ready when the head is firm, and you cut before the stem elongates and the flavor goes bitter. Intense high-elevation sun and a few hot afternoons will bolt lettuce that would have kept going under cloudy eastern skies.
Sow a little every two weeks in spring rather than one huge bed. In summer on the Front Range, use a shade cloth or a bed that gets afternoon shade from a taller crop, and pick cultivars described as slow to bolt. In the mountains you can often sow lettuce all summer, because the bolt signal never quite arrives.
Spinach bolts even faster once days are long. The early sowing, four weeks before the last frost or as soon as soil reaches the high 30s and low 40s, is the one that makes a meal.
A late-summer sowing for fall, and a mulched sowing that overwinters in a Front Range cold frame, will outproduce a May sowing that runs to flower in June. Chard takes the relay. It is hardier in summer heat than spinach and less sweet in cold weather. Grow both, and let each own its month.
Mustard and arugula follow the spinach pattern. Small, repeated sowings. If you sow a ten-foot row once, you will throw away nine feet of it on the day it bolts.
Roots like the cool soil and hate a crust
Carrots, beets, turnips, and radishes are the other half of the hardy list. Radishes are the two-to-four-week crop that tells you the bed is alive. Do not stop there.
The Colorado Vegetable Guide suggests a first carrot sowing in late April in its baseline season, to use soil moisture from snow, and warns that carrot seed can take three weeks and must stay damp the whole time.
A row cover laid on the soil, or a light mist, is the difference between a row and a blank. Thinning is not optional. Crowded carrots are threads.
Clay ground forks carrots. If your soil is the Front Range clay described in clay soils, grow carrots in a raised bed with a deeper, finer mix, or accept shorter cultivars and some forked roots. Beets are more forgiving of texture and give you greens and roots. Turnips and rutabagas belong in the fall where the season allows, and as a summer crop in the high country, where the cool suits them.
The vegetable guide’s note on fall carrots is worth copying where September is real: sow in August in many locations, mulch well, and harvest through cold weather. Roots sweeten in the cool. A mountain August sowing only works if your first frost is late enough for roots to size. Check the station before you copy the August date uphill.
Water, fertility, and the bolt
Cool-season crops are not no-water crops. Dry stress is a bolt signal for lettuce, spinach, and broccoli. Drip on a steady schedule, mulch that will not blow away, and a soil with some organic matter will do more for flavor than a fertilizer spike the week before harvest.
Side-dress brassicas if growth pales, then stop. Lush, late nitrogen grows leaves at the expense of the head and makes a softer plant going into a freeze.
Rotation matters because the cabbage family shares pests and diseases. The vegetable guide’s family table groups cabbage, broccoli, cauliflower, Brussels sprouts, arugula, kohlrabi, turnips, radish, kale, and mustard together.
Do not follow spring broccoli with fall kale in the same square if you can help it. Peas can precede brassicas. They leave the bed early.
Checklist
- Sow the hardy list when soil can be worked, often two to four weeks before the median last 32°F date, or at about 40°F soil at 6 inches in the morning.
- Cover brassicas and arugula immediately for flea beetles and cabbageworm.
- Repeat small lettuce and radish sowings. One sowing is a glut and then a gap.
- Treat spinach as a spring and fall crop on the Front Range, and as a summer crop only where nights stay cold.
- Thin roots. Keep the seedbed damp until carrots emerge.
- Plan a fall brassica or carrot sowing only if your first-frost source says September is open.
- In a garden above 8,000 feet, let this group be most of the square footage.
The warm-season counterpoint is tomatoes, peppers, and squash at altitude. Roots that stay in the ground longer, plus garlic and onions, are in roots, alliums, and potatoes.
Sources
- CSU Extension, Vegetable Planting Guide.
- CSU Extension, Vegetable Gardening in the Mountains.
- CSU Extension, Colorado Vegetable Guide, notes on mountain lettuce and potatoes, carrot timing, cabbage transplants, crop families, and covers.
- NOAA NCEI Colorado freeze-occurrence table, for the station medians cited on the calendar pages.