Planting Calendar for Thornton, CO

Frequently Asked Questions

How Accurate Are the Planting Dates?

Use this chart as a planning guide and a starting place. The frost dates are averages based on historical data; this is NOT a prediction for the coming year. While the majority of gardeners use frost dates as a guideline for planting, some locations in the U.S. do not have frost; in addition, some difficult areas simply don't match up perfectly with the dates provided. Growing in a "microclimate"(such as a low spot or side of a slope) will also affect your planting dates. Watch the weather, talk to fellow gardeners, and take notes on what works in your garden!

Why Do You Start Seeds Indoors?

Starting seeds indoors (in seed trays or starter pots) gives your crops a head start on the growing season, which is especially important in regions with a short growing season. Starting seeds indoors also allows young, tender plants to grow in a stable, controlled environment. Outdoors, the unpredictability of rain, drought, frost, low and high temperatures, sunlight, pests, and diseases can take a toll on young plants, especially when they're just starting. Indoors, you can control these elements to maximize your plants' early growth and give them the best shot at thriving when they are transplanted outdoors. 

For most crops that can be started indoors, seeds should be started about 6-8 weeks before your last spring frost date. This gives the plants ample time to grow large and healthy enough to survive their eventual transplanting to the garden. Read more about starting seeds indoors here. 

Which Seeds Should Be Started Indoors?

Not all vegetables should be started indoors! In fact, most are better off being started directly in the garden (aka "direct-sown"). The crops that should be started indoors are particularly susceptible to cold temperatures or have a very long growing season and need a head start. These include tender vegetables such as tomatoes, peppers, and eggplant and crops with a long growing season, like broccoli, cauliflower, and Brussels sprouts. 

Most other crops do best when sown directly into the garden soil. Root crops, including carrots, radishes, and beets, are especially well-suited to being started directly in the garden since they do not like having their roots disturbed after planting. The same is true for squash and watermelon; though care must be taken to plant them when the soil is warm enough. Read more about direct-sowing seeds here!

When Should You Transplant Seedlings?

It's time to transplant when seedlings have grown too large for their seed trays or starter pots. If it's not yet warm enough to plant outdoors, transplant the seedlings to larger plastic or peat pots indoors and continue care. If outdoor conditions allow, start hardening off your seedlings approximately one week before your last frost date, then transplant them into the garden. Get more tips for transplanting seedlings.

What Is Planting by the Moon?

Planting by the Moon (also called "Gardening by the Moon") is a traditional way to plant your above- and below-ground crops, especially at the start of the season. Here's how it works:

• Plant annual flowers and vegetables that bear crops above ground during the light, or waxing, of the Moon. In other words, plant from the day the Moon is new until the day it is full.
• Plant flowering bulbs, biennial and perennial flowers, and vegetables that bear crops below ground during the dark, or waning, of the Moon. In other words, plant from the day after the Moon is full until the day before it is new again.

Old-time farmers swear that this practice results in a larger, tastier harvest, so we've included planting by the Moon dates in our planting calendar, too. Learn more about Planting and Gardening by the Moon.