When to Plant in Boulder, CO

In Boulder, the gardening year is shaped significantly by its median last spring frost date of May 5 and first fall frost date of October 1, resulting in approximately 149 frost-free days each year. These dates define the safe planting window for tender plants, with growers needing to be mindful of nights that still hold a risk of frost well into spring, which can affect more delicate seedlings and established crops alike.

Boulder's microclimate is characterized by a continental mountain climate, where gardeners experience cold nights and intense sunlight during the day. This unique condition is favorable for crops like tomatoes, peppers, and certain varieties of squash, which thrive in the warm days but require careful timing to avoid night-time chill. Conversely, crops such as leafy greens and some root vegetables can struggle because the intense sun can cause rapid bolting and stress unless managed with shading or appropriate varieties.

What to plant now in Boulder

Spring is the time to plant frost-sensitive crops like tomatoes and peppers after the frost date of May 5. Start these seeds indoors 6-8 weeks prior, transplanting them outdoors in mid to late May.
As summer approaches, it is perfect to sow fast-growing crops such as zucchini and beans. Begin planting these in late May, giving them plenty of sun and warmth throughout June for optimal growth.
In the fall, focus on planting winter-hardy crops like kale and garlic. These should be sown by late September to ensure they establish roots before the first fall frost on October 1.
Winter preparations can include planting cover crops or mulching to protect the garden's soil and improve fertility for the following spring. Start this in late October as temperatures drop and growth slows considerably.

In Boulder (149 frost-free days, a moderate season), 34 of 56 planner crops fit outright, 21 get two plantings, 1 crop is perennial.

Month by month in Boulder

Month	What to do
January	Although gardening is generally quiet in Boulder, this month is ideal for planning your garden layout, ordering seeds, and ensuring your tools are ready for the upcoming season.
February	As winter persists, consider starting your seed trays indoors for crops that will be planted outside after May 5. Monitor the indoor light conditions to suitable seedlings that are robust.
March	March brings longer days, making it an excellent time to prepare garden beds outdoors as well as continuing to nurture indoor seedlings. Start cold-hardy plants like spinach indoors.
April	As April progresses, it's essential to stay vigilant against late frosts. Begin hardening off seedlings started indoors as temperatures start to warm, preparing for outdoor planting around May.
May	May is crucial, marking the end of frost risk with the last expected frost on May 5. Transplant your warm-weather crops outdoors after this date to take full advantage of the growing season.
June	June is a vibrant month in the garden with plants rapidly growing. Continue planting summer crops and monitor for water needs as the weather gets hotter, particularly in full sun.
July	In July, keep an eye on moisture levels and potential pests. Harvest early crops and consider succession planting for fall vegetables, keeping in mind the shortening days.
August	August provides an opportunity to plant late summer crops. This is a great time for sowing fall greens that can mature before the first frost on October 1.
September	With cooler nights arriving, September is when you should plant winter-hardy crops. Start garlic and prepare for the fall harvest while transitioning the garden beds.
October	October signals the end of the growing season with the first fall frost on October 1. Begin cleaning up beds, composting plant waste, and implementing protective measures for overwintering.
November	In November, the garden quiets down as frost becomes consistent. Focus on winter maintenance tasks like mulching to protect the soil and planning for next year.
December	December is the perfect month for reflecting on the year’s gardening success while planning for the upcoming season. Consider ordering seeds and evaluating what worked in your garden.

Frost dates for Boulder ZIP codes

ZIP	Last spring frost (50%)	First fall frost (50%)	Zone
80302	May 5	October 1	6a
80301	May 5	October 1	6a
80303	May 5	October 1	6a
80310	May 5	October 1	6a

FAQ

When is the last frost in Boulder?

The last frost in Boulder typically occurs around May 5, making it important to plan your garden timeline to ensure tender plants are safe from frost.

What vegetables grow best in Boulder?

Summer crops such as tomatoes, peppers, and squash excel in Boulder due to the intensity of the sun during the growing season, while leafy greens tend to struggle.

When should I plant tomatoes in Boulder?

Tomatoes should be transplanted outdoors after the last frost date, around mid to late May, giving them ample time to grow and produce by summer.

Can I garden year-round in Boulder?

While traditional outdoor gardening is limited during winter, you can still garden year-round with cold frames, greenhouses, and indoor plants to enjoy fresh produce.

How does Plan My Garden help Boulder gardeners?

Plan My Garden offers tailored advice, frost date calendars, and gardening tips specific to Boulder’s unique climate, helping gardeners make informed decisions.