When To Plant Squash In Colorado

Knowing when to plant squash in Colorado is the key to a healthy harvest. Get the timing right, and you’ll enjoy a summer full of zucchini, butternut, and other delicious varieties. This guide gives you the clear, local advice you need for success in our unique climate.

Colorado’s weather can be tricky. Our springs are unpredictable, with late frosts and big temperature swings. Planting too early is a common mistake that can ruin your seedlings. But wait to long, and you miss the ideal growing window. Let’s break down the perfect schedule.

For most of Colorado, the safe window for planting squash outdoors is from mid-May through early June. This timing protects your plants from Colorado’s infamous late spring frosts, which can occur well into May.

Here’s a simple rule: Squash plants need warm soil and absolutely no frost. They are tender and won’t survive a chill.

Understanding Your Last Frost Date

Your average last spring frost date is your most important clue. This date varies across the state.

Front Range Urban Corridor (Denver, Boulder, Fort Collins): Average last frost is around May 5-10. Many gardeners target Mother’s Day weekend for planting tender veggies like squash.
Western Slopes & Valleys (Grand Junction, Palisade): Warmer microclimates allow for earlier planting, often in late April.
High Mountain Valleys (San Luis Valley, areas above 7,000 ft): Frost risk can last into early June. Wait until June 1st or later.
Eastern Plains: Dates vary but often align with the Front Range or slightly later due to wind exposure.

Always check your local frost dates. Your county extension office is a great resource for hyper-local information.

The Soil Temperature Test

Frost dates are a guide, but soil temperature tells the real story. Squash seeds germinate best in soil that is consistently at least 70°F. Use a simple soil thermometer to check.

Check the temperature at about 10 a.m.
Insert the thermometer 2-3 inches deep into the soil.
Take readings for several days in a row.

If your soil is holding at 70°F or warmer, it’s go time. If it’s cooler, be patient. Cold, wet soil will cause seeds to rot.

Starting Seeds Indoors vs. Direct Sowing

You have two main options for getting your squash started.

Starting Seeds Indoors

This gives you a head start, especially in shorter-season areas.

When: Start seeds indoors 3-4 weeks before your planned outdoor transplant date. For a May 15th planting, start seeds around April 15th.
How: Plant seeds in individual pots. Squash roots dislike disturbance.
Transplanting: Harden off seedlings for a week before planting them outside. Acclimate them slowly to sun and wind.

Direct Sowing in the Garden

This is often easier, as squash grows fast and doesn’t always transplant well.

Wait until all danger of frost has passed and soil is warm.
Plant seeds 1 inch deep in small mounds or hills.
Space mounds 3-4 feet apart for vining types, 2-3 feet for bush types.
Plant 2-3 seeds per mound, thinning to the strongest seedling later.

Direct sowing avoids transplant shock, which can set plants back a week or more.

Choosing the Right Squash Varieties for Colorado

Some varieties perform better in our climate. Look for those with shorter days to maturity if you live in the mountains.

Zucchini: ‘Black Beauty’, ‘Cocozelle’ (reliable and productive).
Winter Squash: ‘Butternut’ (like ‘Waltham’), ‘Acorn’ (stores well), ‘Delicata’ (shorter season).
Specialty: ‘Sunburst’ Pattypan (early and colorful).

Consider powdery mildew resistance, a common issue in Colorado’s dry yet sometimes humid pockets.

Preparing Your Garden Bed

Squash are heavy feeders. They thrive in rich, well-drained soil.

Choose a Sunny Spot: Full sun (at least 6-8 hours) is non-negotiable.
Amend the Soil: Work in 3-4 inches of compost or well-rotted manure before planting. This improves nutrients and drainage.
Warm the Soil: Using black plastic mulch or landscape fabric a week before planting can raise soil temperature significantly.

Good prep means stronger plants that produce more fruit for you.

Protecting Young Plants

Even after your planting date, Colorado weather can surprise you. Have protection ready.

Frost Blankets or Row Covers: Keep these handy for unexpected cold snaps. They can provide a few degrees of critical protection.
Walls of Water: These plant protectors create a mini-greenhouse effect and are excellent for tender seedlings.
Cloches: Simple milk jugs with the bottoms cut out work in a pinch.

It’s better to have protection and not need it than to lose your plants to a late frost.

Season Extension for Fall Harvest

You can also plant a second, late-summer crop of fast-maturing squash for fall harvest. Aim to plant seeds directly around early to mid-July. This ensures you have fresh squash into the early autumn months before the first fall frost arrives.

Common Mistakes to Avoid

Planting in cold soil. This is the number one reason for poor germination.
Crowding plants. Squash need space for air circulation to prevent disease.
Overwatering at the start. Water seeds well at planting, then avoid soggy soil until seedlings emerge.
Forgetting to harden off indoor-started plants. They need a gradual transition to the outdoors.

FAQ: Planting Squash in Colorado

Can I plant squash in early May in Colorado?

It depends on your location and the year. Along the Front Range, early May is risky. It’s safer to wait until mid-May or use protective covers if you plant early. Always check the 10-day forcast.

What happens if I plant squash too early?

Seeds will rot in cold soil. If seedlings are hit by frost, they will likely die. Even if they survive, cold-stunted plants never catch up to those planted in warm soil.

Is it better to start squash indoors or direct seed?

For beginners, direct seeding after the soil is warm is often simplest. Starting indoors gives a head start but requires careful transplanting. In very short-season areas, indoor starting is recommended.

How often should I water squash in Colorado’s climate?

Water deeply once or twice a week, providing 1-2 inches of water. Focus water at the base of the plant, not the leaves. Our dry air and intense sun means consistent moisture is key, especially once fruit begins to develop.

When can I harvest my squash?

Summer squash (like zucchini) are best harvested small and tender, usually 4-8 inches long. Winter squash are harvested in fall when the rind is hard and the stem is dry and corky, typically before the first hard freeze.

By following these guidelines for when to plant squash in Colorado, you set your garden up for a productive season. Paying attention to soil temperature and local frost dates is more reliable than calender dates alone. With a little planning and patience, you’ll be harvesting basketfuls of homegrown squash all summer long.