Denver Planting Calendar: Month-by-Month Guide for Perfect Timing

Your Denver planting calendar is the key to unlocking a garden that thrives despite the city’s tricky climate—where frost lingers late and summer heat arrives faster than expected.

Denver’s unique elevation, climate shifts, and short growing season demand precision timing for successful gardening—but mastering the perfect planting schedule can double your harvest potential. One wrong move, like planting tomatoes too early, and you’ll watch your seedlings freeze before they even sprout.

Whether you’re a first-time gardener or a seasoned green thumb, knowing when to sow General Denver Vegetables 🌱—from hardy greens in March to heat-lovers in June—makes all the difference. This guide breaks down the exact months to plant, what to avoid, and how to stretch your season with simple hacks.

Ready to grow more with less guesswork? Here’s your step-by-step roadmap to a bountiful Denver garden, tailored to Zone 5b-6a and beyond.

Denver’s frost dates and planting zones: when to start seeds indoors vs. outdoors

Denver’s high elevation and dramatic temperature swings create a gardening challenge—but also a golden opportunity for those who time their plantings just right. With an average last spring frost on May 15 and first fall frost around September 20, you’ve got roughly 120 frost-free days to work with.

That’s why understanding your USDA Zone 5b/6a and when to start seeds indoors versus outdoors is the difference between a thriving garden and a sad, frost-nipped failure.

Let’s break down the critical timing for your cool-season crops like spinach and peas versus heat-loving plants such as tomatoes and peppers.

The key is knowing when to begin seeds indoors to give them a head start, and when to sow directly outside for maximum success in Denver’s short but sweet growing season.

Denver’s frost dates and planting windows are your gardening compass. The last frost date (May 15) signals the start of outdoor planting for warm-season crops, while the first frost date (Sept 20) tells you when to harvest or protect tender plants. For Zone 5b/6a, this means planning ahead—especially for crops that need a long growing season.

Here’s a quick comparison to help you decide when to start seeds indoors versus outdoors, tailored to Denver’s unique climate:

Starting seeds indoors for warm-season crops like tomatoes and peppers gives them the 8-10 weeks they need to mature before Denver’s last frost. For example, tomatoes need to be indoors by March 15 to be ready for transplanting around May 15.

Meanwhile, cool-season crops like spinach and peas can often be sown directly outdoors as early as March 15, as long as you protect them with row covers if frost threatens.

Denver’s Zone 5b/6a classification means you’re in a transitional zone where both early spring frosts and late summer heatwaves can catch you off guard. That’s why I always recommend having a cold frame or row cover on hand to extend your growing season.

For instance, you can start lettuce indoors in February and transplant it outside by March 15, then use a row cover to shield it from late frosts.

One of my biggest mistakes early on was waiting too long to start seeds indoors. I learned the hard way that peppers, in particular, need a long growing season and won’t mature if planted too late.

Now, I mark my calendar with March 1 as the deadline for starting peppers indoors to ensure they’re ready by May 20. Similarly, tomatoes should be indoors by March 15 to avoid rushing their growth.

For cool-season crops, the strategy shifts. You can sow spinach, peas, and carrots directly outdoors as early as March 15, but be prepared to cover them if temperatures dip below 40°F.

These crops thrive in cooler weather and can even handle light frosts, making them perfect for Denver’s early spring. Just remember: succession planting is your friend here—sow every 2 weeks for a continuous harvest.

Denver’s first fall frost on September 20