When To Plant Cool-Season Vegetables In High-Altitude Wyoming Gardens

When you garden at high elevation in Wyoming you are gardening in an environment defined by short growing seasons, wide diurnal temperature swings, intense sunlight, and frequent late or early frosts. Timing sowing and transplanting for cool-season vegetables is different here than in lower-elevation, longer-season areas. This article explains how to schedule plantings using frost dates, soil temperatures, days-to-maturity, and practical season-extension techniques so you reliably grow lettuce, peas, brassicas, root crops, and other cool-season favorites.

High-altitude realities that determine planting time

Growing conditions at 6,000 to 9,000 feet (and higher) in Wyoming typically include the following constraints and advantages. You must plan around them.
Cold and frost risk
Frosts are common late into spring and can return early in fall. Many plant hardiness maps place most of Wyoming in USDA zones 3 to 5; local microclimates can shift this by a zone. Expect last spring frosts from late May into July depending on elevation and exposure, and first fall frosts from August to October.
Short, but intense, growing season
The number of frost-free days often ranges from roughly 60 to 120. Select vegetables and varieties based on days-to-maturity rather than calendar dates alone.
Wide diurnal swings and strong sunlight
Warm daytime temperatures can be followed by very cold nights. Intense UV and reflected heat from snow or rock can lead to rapid plant growth during warm spells but can also cause bolting or sunscald.
Soil and drainage
Cold, compacted, or rocky soils warm slowly in spring. Raised beds and well-drained soil enable earlier planting and better root development.

Use frost dates, not just calendars

The single most useful reference for timing is your local average last spring frost and first fall frost. But at high elevation those dates vary widely across short distances. Do this:

Find your last and first frost estimates from a reliable local source (county extension, local gardeners, or weather stations). Treat them as a range rather than an exact day.
Use “weeks before or after last frost” rather than fixed calendar dates. For example: “sow peas 4 weeks before last frost” instead of “sow peas in April.”
Combine frost-date timing with soil temperature guidance and days-to-maturity for each crop.

Soil temperatures to watch

Soil temperature matters for germination and early root growth. Use a soil thermometer and wait for these approximate thresholds for direct sowing:

Peas and spinach: 35 to 45 F (can germinate at very cool temps, best above 40 F).
Lettuce and radish: 40 to 45 F (germinate faster as soil warms).
Carrots and parsnips: 45 to 55 F.
Beets and chard: 45 to 50 F (beets will germinate cooler but slowly).
Brassicas (broccoli, cabbage, kale): 45 to 55 F for transplant roots; you can set out hardened transplants when soil is in this range even if light frosts remain.
Onions (sets): 40 to 45 F; leeks start indoors.

If your raised bed soil reaches these temperatures earlier than native ground, you can sow there sooner.

General planting rules for cool-season crops

Below are practical rules that translate frost dates, soil temps, and plant maturity into actions.

Start broadleaf and leaf crops early.
Direct-sow or transplant lettuce, spinach, arugula, and mustard as soon as the soil is workable and above about 40 F. These crops tolerate light frosts and can be planted 3 to 4 weeks before the average last frost if you protect them with row cover on cold nights.
Sow peas early.
Peas can be sown 4 to 6 weeks before last frost in many high-elevation sites. They are adapted to cool soil; if a late freeze occurs, they typically recover.
Hard transplants for brassicas.
Start broccoli, cabbage, and cauliflower indoors 6 to 8 weeks before last frost. Harden them and transplant outdoors 2 to 3 weeks before last frost–brassicas tolerate frost and often perform better when given a head start in cool weather.
Root crops slightly later or use warmed beds.
Carrots, beets, and parsnips germinate slowly in cold soils. If soil remains below 45 F, use black plastic to warm the seed bed or wait until soil temps reach the thresholds above. Sowing 1 to 2 weeks before last frost is common where soils are warm enough; otherwise sow immediately after last frost for a main-season crop.
Succession plant frequently.
For continuous harvests of leafy greens and radishes, sow small batches every 7 to 14 days while conditions permit.
Plan a fall planting.
In high-altitude Wyoming, many cool-season crops planted in mid-to-late summer will give a productive fall harvest before hard freezes. Start fall sowings based on days-to-maturity and your first expected fall frost.

A practical planting timetable relative to last frost

Use this as a guideline. Adjust for your specific frost dates, elevation, and microclimate.

6 weeks before last frost: Sow peas (early varieties) outdoors if soil workable; start brassica transplants indoors.
4 to 5 weeks before last frost: Direct-sow spinach, radish, and cold-tolerant lettuce in protected beds; sow early root crops in warmed soil.
3 to 4 weeks before last frost: Transplant hardened brassica seedlings outdoors; sow another round of lettuce/greens for succession.
1 to 2 weeks before last frost: Direct-sow beets, carrots (if soil >=45 F), chard, and onion sets.
At last frost to 2 weeks after: You can safely direct-sow most remaining cool-season crops and transplant warm-season seedlings only when soil is significantly warmer and danger of frost has passed.

Fall plantings and overwintering

Many cool-season vegetables do a second act for fall harvest. To succeed:

Count back from the average first fall frost by the crop’s days-to-maturity plus an extra 10 to 14 days buffer. For example, if your first frost is mid-September and you have a spinach that matures in 35 days, plan to sow around late July or early August.
Use floating row cover or low tunnels to extend the harvest into late fall. Kale, collards, and certain cabbages can withstand light freezes and improve in flavor after a frost.
Consider overwintering varieties for spring greens. In some protected locations, spinach and certain hardy lettuces overwinter and resume growth early the next spring.

Season-extension techniques that make early planting practical

High-elevation gardeners use several effective tools to shift the growing window earlier and later in the season.

Raised beds: Warm faster in spring and drain better.
Black plastic or fabric row cover on soil: Warms seedbeds for faster germination.
Floating row covers: Protect seedlings and transplants from light frost and wind; they raise air temperature under cover by several degrees.
Low tunnels and hoop houses: Useful for both spring startup and fall extension; they prevent hard freezes on many cool-season crops.
Cold frames and cloches: Excellent for hardening off seedlings and for early planting of salad greens and herbs.
Mulch and straw: Use straw to insulate soil for overwintering transplants or to moderate soil temperature in early spring if frost heaving is a concern.

Variety selection and maturity considerations

At high elevation, days-to-maturity is a critical selection criterion. Favor:

Fast-maturing varieties (shorter days-to-maturity) for the main season.
Cold-hardy selections for spinach, kale, and brassicas.
Varieties marketed as “early” or “short season” for peas, carrots, and beets.
Heirloom vs hybrid: Both have roles. Hybrids often mature faster and more uniformly, which can be helpful when the window is tight.

Also look for disease resistance relevant to your area (downy mildew on lettuce, fusarium in brassicas) to reduce loss of early plantings.

Hardening off, pests, and watering considerations

Harden off seedlings slowly over 7 to 10 days to acclimate them to high-intensity sun and cold nights. A sudden transplant from warm indoor conditions into bright, cold mountain sun can stress and scorch foliage.
Water early in the day so plants acclimate and leaves are dry at night; this reduces fungal risk.
Watch for cutworms, flea beetles, and slugs that can damage early plantings. Row covers protect both from cold and some pests.
Use light shade cloth on very sunny spring days for tender seedlings to prevent sunscald.

Common mistakes to avoid

Planting only by calendar date: In high-elevation Wyoming you must use frost-range and soil temperature cues.
Ignoring soil temperature: Seeds sown into cold soil either rot or germinate erratically. Test the soil.
Overestimating your growing season: Select varieties with comfortable maturity windows and have fall plans.
Skipping succession sowing: A single sowing of lettuce will not provide a season-long supply.

Practical takeaways

Use last and first frost dates as ranges and schedule plantings relative to those dates.
Measure soil temperature before direct sowing; wait for the crop-specific thresholds listed above.
Sow peas and some greens very early; start brassicas indoors and transplant before last frost; delay slow-germinating root crops until soil is warm enough or warm the bed first.
Use raised beds, row covers, low tunnels, and mulch to shift your season earlier and later.
Choose fast-maturing and cold-hardy varieties and practice succession sowing to make the most of the short mountain growing season.

High-altitude Wyoming gardens reward careful timing and strategic season extension. When you match sowing dates to soil temperature, frost risk, and realistic days-to-maturity, you will harvest reliable, flavorful cool-season crops even in challenging mountain climates.