WHEN AND HOW TO START SEEDS FOR YOUR 

COLORADO GARDEN

By CINDY SUTTER | Special to The Denver Post

PUBLISHED: February 10, 2017 at 8:00 pm | UPDATED: February 12, 2017 at 2:09 pm

With the balmy weather Colorado has experienced this past week, eager gardeners could be forgiven for thinking it’s time to plant their spring and summer vegetables.

As more seasonal temps with freezing nights return in the coming week, reality will alas intrude, as it tends to do. But that doesn’t mean you can’t scratch that gardening itch. You can ramp up your seed-starting operation for the season.

How to get started with seeds

If you’ve never started seeds, not to worry. It’s not that complicated. Think of it this way: When agriculture started 10,000 years ago, our whole species didn’t know how to grow food, but they caught on fast. Now, with the wisdom of millennia to tap into, you’ve got this. Here’s how.

Tagawa Gardens adviser Linda Larsen offers a key piece of advice to those who have never gardened before: “I would decide by what I have space for.”

In other words, don’t buy seeds for 50 different veggies if you only have room for a garden that’s 10 feet by 10 feet. The other thing to remember as you’re choosing a garden site is that you need full sun. If a tree shades the spot in the summer, it won’t work.

With that in mind, pick the seeds you want to start and separate them by the season in which they are grown and harvested. In general, cool season crops include greens, lettuce, peas, radishes, broccoli, cauliflower, turnips, beets and carrots. Warm season crops are those harvested in mid-to-late summer, such as tomatoes, peppers and eggplant.

Making things somewhat more complicated is that some veggies should be started inside and others should be directly planted into the ground. How do you tell the difference? Look at the instructions on the seed packet, Larsen advises.

“The packages contain a wealth of information,” she says.

Many gardeners prefer to directly sow the cool crops, rather than starting them inside. (That’s what Larsen prefers.) However, for those who want to get a bit of a jump on the season or just can’t wait to get their hands into some dirt, cool-season crops such as greens or lettuces can be started inside now and transplanted into the garden in March or April.

Paul Cure, who, with his wife, Anne, owns Cure Organic Farm in Boulder, says they have head lettuces growing in their greenhouse now. The head lettuce is destined for student gardens at Boulder Valley Schools.

Cure, whose farm will sell its produce at the farmer’s markets at Denver Union Station and in Boulder, says greens and head lettuces can be started inside now or direct-sown outside in mid- to late March if you have row cloth to protect them from the coldest temps. Greens such as kale, collards and mustards, are among the vegetables best suited to cold. They can take a bit of frost and the nip of cold makes them sweeter. Hardiness temperatures are generally listed on the seed packet.

Plan first

Average frost date in Colorado

Open a seed packet in a search for information, and you may find yourself still a bit befuddled. Instructions for sowing outside or starting inside almost always refer to the average frost date — as in “Start inside 6 to 8 weeks before average frost date.”

The reason for writing the directions that way is that frost dates vary by climate. Your friend in Georgia may be able to set out tomatoes in early April, while your uncle in Minnesota may have to wait till the end of May or even early June.

In most places in Colorado, the average frost date is between May 15 and May 30. But certain variables complicate the picture. The higher the altitude, the later the frost date, since warm spring temperatures come later in the mountains.

Larsen says that for most of Denver proper, the average frost date is about May 15. Some families even have a tradition of planting warm weather vegetables plants, such as peppers and tomatoes, on Mother’s Day. Larsen, says however, that some places outside Denver may see a later average frost date. She lives near Parker and considers her average frost date to be May 25. Some gardeners hold off until Memorial Day weekend.

Days to maturity

Colorado is a tough place to garden because the growing season is short. If you plant tomatoes in late May, they could face a frost in mid-September. That means it’s important to know how long it takes from planting to harvest. Some crops are quick: For example, certain varieties of radishes, a cool season crop, take only 30 days from seed planting to eating. Many tomatoes, especially the larger beefsteak varieties, take 90 days or more.

Seed-starting equipment

Most people buy oblong plastic trays with domes from a garden center for their seed starting. The trays prevent the water that drains from pots from dripping, and the domes create a greenhouse effect. Larsen also recommends a germination mat, a sort of soil heating pad that goes under the trays to keep the temperature warm. The trays, domes and mat are reusable for the next year. Rather than using a germination mat, some gardeners keep their seed trays warm by placing them on top of the refrigerator or next to their furnace.

“There’s not a seed around that likes to germinate if they don’t at least have a temperature of 55 degrees,” Larson says.

Some seeds, such as peppers, like it positively tropical.

For individual pots, many people use four-pot and six-pot inserts that fit into the trays, Larsen says. But gardeners looking to save money use paper cups, egg cartons or even plastic ice cube trays, all with drainage holes cut into them.

For best results, gardeners should use a soil medium instead of garden soil or potting soil for seed starting. Most nurseries have their own soil medium for sale. At Tagawa Gardens, the medium is made with perlite, vermiculite and peat moss. The lightweight medium with the moisture-holding peat allows seeds to sprout and roots to form without having to work very hard.

How to plant seeds

To plant seeds, Larsen moistens the soil medium and places it into the pots. She uses a chopstick or skewer to make a small hole in the medium and drops in two seeds per pot. She uses a spray bottle to mist the seeds to keep them moist, checking them daily. If both seeds germinate, she snips off the smaller plant, once both have developed their first two true leaves (the plant’s second set of leaves after the small leaves that emerge after germination).

Another key point: Make sure you label the pots with markers, so you know what is in each pot.

When true leaves have emerged, the plants should be watered regularly and given a glug of half-strength liquid fertilizer each week.

After a good mass of roots has grown, the seedlings should be transplanted into larger pots with potting soil. Larsen advises a second transplant after they outgrow the first pot with soil.

Shining a light on seedlings

While some plants can get by with good natural light, warm-weather plants such as tomatoes that may spend a couple of months indoors do best with grow lights. Many gardeners buy shop lights and use warm (red) spectrum tubes alternated with cool (blue) spectrum light. Or gardeners can buy daylight spectrum tubes.

Many gardeners hang their light setup on chains, keeping the fluorescent lights close to the plants and raising them as the plants grow.

Other tips for keeping your veggie starts happy

Larsen says to water the plants with room temperature water. When transplanting, grab the plant by the leaves rather than the stem, she adds.

“If you bruise a leaf, especially a lower leaf, it’s not a big deal,” she says. “If you bruise a stem, it could alter that plant for the rest of its life.”

When it’s time to plant your babies outside, get them ready by exposing them to the outside temperature and light for increasing periods of time each day, making sure they’re well watered.

Keep an eye on the weather and future weather reports. If the nights have been cold and the soil is less than 50 degrees, warm weather plants may be stunted. Putting dark plastic or landscape fabric on the outside beds can warm the soil.

Why do it?

You may be thinking that seed starting seems like a lot of trouble when you could buy plants from a garden center or even buy vegetables from the farmer’s market.

You like doing it.

Here’s another argument for those who have children: It’s a great family project, says Cure, of Cure Organic Farm.

“It’s always such an incredible process to see the life of a seed, how a plant can grow from something so small,” he says. “I’m always amazed.”

When to plant in Colorado

For the most specific information, refer to your seed packet.

In general:

Cool-weather seeds can be started inside now through the end of the month. Depending on the cold tolerance of the plant, they can be sown outside starting in mid-to-late March.

Warm-weather seeds should be started in mid-March. Many Coloradans remember St. Patrick’s Day as the time to start seeds. Warm-weather seeds for tomatoes, peppers and eggplants cannot be sown outdoors after the mid-to-late May frost date — they won’t have time to mature. But summer and winter squash, cucumbers and green beans can be sown after the frost date when the soil is warm.