Carol O'Meara - Colorado State University Extension

Carol O’Meara – Colorado State University Extension

As a teacher of gardening, there are times when Mother Nature drives me crazy.  Just as I’m explaining to classes that here in the Rocky Mountains soil temperatures are every bit as important as frost dates for planting, she sends freezing rain, snow and hail after Mother’s Day as if to punctuate my lessons.

Maybe she thinks she’s being helpful; after all, we needed the moisture and she perfectly illustrated my point that warm season plants will sulk if the soil is too cold for them. But her teasing us with warmth in between these storms is beyond cruel: rumors of impromptu smudge pots and space heaters being deployed in the garden make me believe gardeners tipped towards insanity when the cold moved in.

Fortunately the warmth seems settled in. But after a week where temperatures hovered around 55 degrees or below, the plants need a signal to actively resume growth.  To give your vegetables, annuals, and perennials that stimulus, fertilize them.

If you’re growing cabbage, kale, broccoli, cauliflower or other cole crops and haven’t fertilized them since planting, apply fertilizer now and again in two weeks. Rapid growth and a steady supply of water is the secret to keeping them sweet. They’ve gotten the water; they need the fertilizer at this time.

Warmer season favorites, like tomatoes, eggplant or peppers, should be given a boost of fertilizer so they shrug off the effects of the cold weather.  Although the soil isn’t completely warmed up – it’s hovering around 60 degrees at my house and on campus in Fort Collins – the predicted warmth should dry out and warm the soil rapidly.

Delay fertilizing potatoes until six weeks after they’ve nosed up from the ground and you’ve finished hilling them. Applied too soon, nitrogen will delay tuber set, and you’ll end up with much smaller potatoes at harvest.  Usually six to seven weeks after they’ve been hilled is a good time to fertilize potatoes. Break up the application into two-week intervals and apply it three times.

Onions respond well to a schedule of fertilizing every two weeks. With shallow roots, they’re not able to reach deeply into the soil to access nitrogen held below their roots. If you’re hoping to get big, bulbous onions, feed them every other week.

Once you’ve given a quick boost of fertilizer to any vine crops that didn’t perish under the snow, watch for when the vines send out runners, then side dress them with nitrogen once more.  After that, back off on fertilizing to avoid getting vines big enough to rope in toddlers without having any squash.

What you use to fertilize the garden is up to you. Keep in mind that we have plenty of phosphorus and potassium in our soils and focus on providing the plants with nitrogen. Bat guano, blood meal, and fish meal all provide at least 10 percent nitrogen, while fish emulsion, cottonseed meal, and alfalfa meal have lower amounts.  Adjust your fertilization rates based on the plant’s performance and get a soil test if you’d like to know how fertile to soil in your garden is.

By Carol O’Meara, Colorado State University Extension. Colorado State University Extension, together with Boulder County Parks and Open Space, provides unbiased, research-based information about consumer and family issues, horticulture, natural resources, agriculture and 4-H youth development. For more information contact Extension at the Boulder County Fairgrounds, 9595 Nelson Rd., Box B, Longmont, 303.678.6238, e-mail [email protected] or visit