It's also time to direct seed indoors certain cool season vegetables, especially if you're using cold frames for earlier planting in March. Check out the planting chart below.
For newbies to Colorado there are three overlapping seasons to plant outdoors.
- The first cool season planting period ranges anywhere from March to the middle of May. These include cool weather loving vegetables like spinach, peas and beets and cool season annuals like pansy, calendula and sweet peas.
- The warm season window is anywhere from mid-May to the first of July and includes vegetables like - tomatoes, peppers, squash and corn, herbs like basil, plus annuals - petunias, marigolds, sunflowers and cosmos.
- Hardy perennials, shrubs and trees can be planted during the warm season period and all the way to early fall. The exception is to try to avoid planting when temperatures are extremely warm (85+). It can be done, but pay close attention to watering and providing some shade for a few weeks.
- Exceptions - bare root roses, plus bare root trees and shrubs can be planted almost anytime the soil is workable (not too wet) from March to late April or so.
- Mid-summer is when the
third season planting window begins - mostly cool season vegetables that
mature in sixty days or less and warm season crops that also have a
shorter maturity date like summer squash, okra and basil.
The timing and planting information on seed packets vary per company. Some mention soil temperatures or map zones as guides to direct seed outdoors, or a certain number of days from frost dates to start seeds indoors. Familiarize yourself with the seed packet information. Check out this helpful article - Garden Primer How to Read a Seed Packet. I'm suggesting to use May 15 as the probable final spring frost date, but we know not to bet the house on that date. In 2007 the last spring frost was June 8.
If you miss the window to start your seeds indoors, you can always purchase cool season transplants at garden centers starting in late winter into spring, and later in the spring for warm season plants.
Warm season seeds are generally started indoors in April for late May transplanting. I'll post the warm season and herb planting charts soon.
Please be aware that some cool season vegetables prefer being directly seeded outside in soil and not started inside, like arugula and other greens. Whereas some vegetables can be seeded indoors for transplanting later outdoors or directly seeded outside, like kohlrabi. Some vegetables are perennial, so chose your site carefully because they won't like being moved. Click here for early - soil preparation.
Cool Season Vegetable Seeding and Transplanting Chart
(a few herbs are included):