When To Plant Watermelon In Colorado

If you want to grow watermelon in Colorado, timing is everything. Knowing exactly when to plant watermelon in Colorado is the first step to a succesful harvest.

Our short summers and unpredictable weather make gardening a challenge. But with the right schedule, you can enjoy sweet, homegrown melons. This guide gives you the clear, practical steps you need.

This is your most important date. Watermelons are extremely sensitive to cold. Planting too early is a common mistake that can ruin your crop.

The best time to plant watermelon seeds or transplants outdoors is 2 weeks after your area’s average last spring frost date. The soil must be warm—at least 70°F—for seeds to germinate and plants to thrive. Cold soil will stunt growth or cause seeds to rot.

Understanding Colorado’s Frost Dates

Colorado’s climate varies dramatically. Your planting date depends entirely on where you live.

Front Range Urban Corridor (Denver, Boulder, Fort Collins): Average last frost is around May 5-15. Aim to plant between May 20 and early June.
Western Slopes (Grand Junction, Palisade): This area warms up earlier. Last frost can be mid-April. Planting in early to mid-May is often safe.
High Mountain Valleys (San Luis Valley, areas above 7,000 ft): Frost risk can persist into early June. Wait until mid-June to plant, and choose short-season varieties.
Eastern Plains: Dates are similar to the Front Range, but wind is a bigger factor. Use row covers for protection.

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

Starting Seeds Indoors vs. Direct Sowing

You have two main options for getting your watermelons started. Each has it’s pros and cons.

Starting Seeds Indoors:

Gives you a head start, which is crucial for long-season varieties.
Start seeds 3-4 weeks before your planned outdoor planting date. Use biodegradable pots to avoid disturbing roots.
Do not start them to early, as watermelon seedlings get root-bound and stressed quickly.

Direct Sowing in the Garden:

This is simpler and avoids transplant shock.
Wait until soil is truly warm. You can pre-warm soil using black plastic mulch for a week or two before planting.
Plant seeds 1 inch deep in small hills or rows.

Choosing the Right Watermelon Variety for Colorado

Not all watermelons are created equal for our climate. Focus on varieties with a “days to maturity” of 85 days or less. Here are some reliable choices:

Sugar Baby (80 days): A classic icebox type. Small, round, and deep red. Very reliable.
Blacktail Mountain (70-75 days): Developed for short seasons. Surprisingly large fruit for its early maturity.
Yellow Doll (68-70 days): A yellow-fleshed, sweet variety that matures quickly.
Faerie (75 days): Unique with yellow rind and pink flesh. Performs well in shorter seasons.

Avoid giant, 100+ day varieties like Carolina Cross. They simply won’t ripen before our first fall frost.

Step-by-Step Planting Guide

Prepare the Soil: Choose the sunniest spot in your garden. Watermelons need 8+ hours of direct sun. Work in 3-4 inches of compost or well-rotted manure. Soil should be well-draining.
Warm the Soil: Lay black plastic mulch down 1-2 weeks before planting. This is a game-changer in Colorado.
Plant Your Seeds or Transplants: For hills, space them 6 feet apart. Plant 3-4 seeds per hill, thinning to the strongest 2 plants later. For transplants, gently tear the bottom off peat pots and plant them pot and all.
Water Gently: Water thoroughly after planting. Use a gentle spray to avoid washing away seeds.
Protect from Cold: Have floating row covers or cloches ready for unexpected late cold snaps. Remove them once daytime temperatures are consistently above 70°F.

Caring for Your Watermelon Plants

Consistent care is key once your plants are established.

Watering: Water deeply 1-2 times per week, providing 1-2 inches of water. Avoid overhead watering to prevent leaf diseases. Drip irrigation is ideal. Reduce watering slightly as fruit ripens for sweeter melons.

Fertilizing: Use a balanced fertilizer at planting. When vines start to “run,” switch to a lower-nitrogen, higher-phosphorus fertilizer to encourage flowering and fruiting.

Weeding: Weed carefully when plants are young. Once vines spread, their dense leaves will shade out most weeds.

How to Tell When Your Watermelon is Ripe

Harvesting at the right time is crucial. Here’s how to know:

The Curly Tendril: Look at the tendril closest to the fruit stem. When it turns brown and completely dries up, the melon is usually ripe.
Ground Spot: The spot where the melon rests on the ground changes from white to a creamy yellow.
Sound: A ripe melon will have a deep, hollow sound when you thump it (though this takes practice).
Sheen: The rind loses its shiny gloss and becomes dull.

Do not rely solely on the size or the rind color of the variety. Use at least two of the signs above.

Common Problems and Solutions in Colorado

Cool Weather: This is the biggest hurdle. Use season extenders like walls of water, cloches, and black plastic mulch religiously.
Cucumber Beetles: They spread bacterial wilt. Use row covers until plants flower, then remove for pollination.
Powdery Mildew: Common in late summer. Plant resistant varieties and ensure good air circulation. Avoid wetting leaves when watering.
Poor Fruit Set: If flowers aren’t turning into fruit, it’s often due to cool weather or lack of pollinators. You can hand-pollinate flowers using a small paintbrush.

FAQ: Growing Watermelon in Colorado

Q: Can I plant watermelon in pots in Colorado?
A: Yes, choose a compact “bush” type variety and use a very large container (at least 20 gallons). Ensure it has excellent drainage and gets full sun all day.

Q: What if I planted my watermelons to early?
A: If a light frost is forecast, cover them immediately with row covers or blankets. If plants are stunted from cold, they may recover when weather warms, but you might have a shorter harvest window.

Q: How long does it take for watermelons to grow?
A> From planting to harvest, most short-season varieties take 70-85 days. Always count from when you transplant or the seeds sprout outdoors, not from when you started seeds indoors.

Q: Should I prune my watermelon vines?
A: Generally, no. Colorado’s season is to short to risk reducing the leaf area that feeds the fruit. Let the vines run freely.

Q: What’s the best watermelon for high altitude gardening?
A: Stick with the shortest-season varieties like ‘Sugar Baby’ or ‘Blacktail Mountain.’ Using black plastic mulch and wind protection is even more critical at altitude.

Growing watermelon in Colorado is a rewarding test of your gardening skills. By focusing on the right planting time, choosing fast-maturing varieties, and using simple techniques to warm the soil, you can beat the clock. Pay close attention to your local conditions and be ready to protect your plants. With a little planning, you’ll be enjoying the taste of summer from your own garden.