With the first couple of months of the new year behind us, gardeners are focused on one thing: the countdown to the outdoor growing season.

Vegetable seeds that need to be started indoors very soon for transplanting outside in March to April include broccoli, Brussels sprouts, cabbage, parsnips, onions and cauliflower. (Betty Cahill, Special to The Denver Post)
Vegetable seeds that need to be started indoors very soon for transplanting outside in March to April include broccoli, Brussels sprouts, cabbage, parsnips, onions and cauliflower. (Betty Cahill, Special to The Denver Post)

Because spring weather in Colorado can be all over the map with moisture and temperatures, we know that dates don’t necessarily equate to the exact time or window to start planting outdoors. However, there are some flexible rules to follow so that we won’t be left behind.

For newbies (and as a review for more experienced gardeners), there are three defined and overlapping outdoor growing seasons to put plants and seeds (based on type) in the ground along the Front Range. For higher elevations, adjust the spring and fall seasons by waiting longer in the spring and plan for a shorter fall growing season.

The indoor seeding season is based on timing, plant type, and what the seed packet says about the time needed to grow indoors under lights or near a sunny window before planting them outside.

In a nutshell: learn it (plant type you wish to grow); live it (plant in the correct window); love it (eat the fruits and vegetables and enjoy every bloom that you plant).

Planting windows overview  

The cool-season planting window ranges anywhere from March to the middle of May. These include cool-season vegetables, which prefer growing in cooler daytime temperatures and are tolerant of cooler nighttime temperatures. (Betty Cahill, Special to The Denver Post)
The cool-season planting window ranges anywhere from March to the middle of May. These include cool-season vegetables, which prefer growing in cooler daytime temperatures and are tolerant of cooler nighttime temperatures. (Betty Cahill, Special to The Denver Post)

This planting and seeding overview shows the big picture of the cool and warm-season seeding indoor, outdoor and planting for all three planting windows.

1. The cool-season planting window ranges anywhere from March to the middle of May. These include cool-season vegetables, which prefer growing in cooler daytime temperatures and are tolerant of cooler nighttime temperatures. They include spinach, lettuce, peas, potatoes and beets, and cool-season annuals like pansy, calendula, snap dragon and sweet pea.

If snow is on the ground or it’s raining or snowing from mid-March to mid-May, the cool- season planting window may either be delayed or skipped; it’s your choice. The good news is you can plant most cool-season plants in mid-summer (season 3) for fall eating.

Tunnels, cold frames, row covers and cloches are recommended to help you through uncooperative weather.

2. The warm-season window is anywhere from mid-May to the middle of July. This group of plants must have warmer daytime and nighttime temperatures to grow and mature. They are intolerant of frost and temperatures in the 30s and do not grow well in the 40s. These include ornamental perennials, vegetables such as tomatoes, peppers, squash, corn and basil (herb), plus annuals such as petunias, geraniums, marigolds, sunflowers and cosmos.

3. Mid-summer (mid-July to August), also called the fall planting season, is when the third season planting window begins. This includes 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.

This is also a great time to plant perennials that were missed earlier. The selection may be more sparse, but many perennials begin going on sale later in the summer into fall.

Indoor seeding charts

For seeding plants indoors, the windows and tips can be found on one of the four charts linked here. They include indoor seeding for ornamental annuals, perennial and annual herbs, and cool and warm-season vegetables. Not included is a perennial indoor seeding chart, although many perennial plants can be seeded indoors while others are only available as grown plants in garden centers and online.

The timing, seeding and planting information on seed packets can vary per company. Some mention soil temperatures or map zones as guides to direct seeding outdoors, or a certain number of days from frost dates, to start seeds indoors. Pay attention and familiarize yourself with the seed packet information. The charts here try to accommodate most seed company brands.

Seed packets will give you instructions on how and when to plant, like these leek seeds. (Betty Cahill, Special to The Denver Post)
Seed packets will give you instructions on how and when to plant, like these leek seeds. (Betty Cahill, Special to The Denver Post)

Vegetable seeds that need to be started indoors very soon for transplanting outside in March to April include broccoli, Brussels sprouts, cabbage, parsnips, onions and cauliflower.

Tunnels or cold frames are recommended for early planting when freezes and frosts are the norm. Soil conditions for setting out new plants or direct seeding need to be spot on, too — not too wet, frozen or cold.

Warm-season seeds including tomatoes and peppers are generally started indoors in late March to April for mid- to late-May transplanting.

Keep in mind that if you miss the window to start your seeds indoors, you can always purchase cool-season transplants at garden centers in February and well into spring. Circle back later in April and May to purchase warm-season transplants.

Trees, shrubs, lawns

Most shrubs and trees can be planted as early as March and all the way to early fall. The exception for woody plants and perennials is to try to avoid planting when temperatures are extremely warm — unless it’s a scheduled installation or a must for your landscape. It can be done, but pay close attention to watering and providing some shade for several days after being planted if possible.

Turf grass can be seeded or sodded beginning in spring. Seed cool-season grasses including bluegrass, fescue and ryegrass anytime from March through September. Seed warm-season grasses like blue grama, buffalo grass and Bermuda grass from April through July. Avoid seeding after early August so the seed has plenty of time to get established.

People living at 7,000 feet elevation or higher should seed or sod cool-season grasses when the temperatures remain warmed up in the spring and before fall frost sets in. Warm-season grasses at high elevations can be seeded in late June to early July if typical monsoon rains are present over a period of time to help the seeds get going. Or seed in late fall so that the seeds will germinate the following spring.

Resources

Building Inexpensive Hoop Houses Video with Betty Cahill:  Part 1  and  Part II

Frost dates and climate summaries for Colorado: cmg.extension.colostate.edu/gardening-resources/online-garden-publications/frost-frost-dates-and-climate-summaries/

High-altitude native grasses 

 

 Denver, CO - MARCH 15: Denver Post garden contributor Betty Cahill demonstrates how to properly divide and move plants for this week's DPTV gardening tutorial. Plants are divided or moved because they are overgrown, overcrowded, lack vigor or are in the wrong place. Spring is the best time to move summer and fall blooming plants. (Photo by Lindsay Pierce/The Denver Post)

Betty Cahill is a freelance writer who speaks and writes about gardening in the Rocky Mountain Region. Visit her site at http://gardenpunchlist.blogspot.com/ for even more gardening tips.