Microclimates Matter

nga-app-planting-guideThe link promised to tell me when I should start my vegetable garden—when to sow seeds indoors, and when to sow or transplant outdoors. Just type in my zip code, and I’d have information customized for my area, courtesy of the National Gardening Association. I rarely click on ads, but I’ve found the NGA to be helpful in the past. Besides, I was curious. I have 24 years of records telling me when to plant in my area—how would their site measure up?

My zip code is 80924—the north end of Colorado Springs. The first thing I noticed was that the website said “Colorado Springs,” and the little Google maps marker was pegged at the airport. Not a good sign.

I live at 7,100 feet, and my garden beds occupy a windy site on a hillside with a southern exposure. The airport is at the southeastern corner of the city, nearly a thousand feet lower in elevation, and is flat. Temperatures at the airport are much warmer than here in my yard. Still, I was surprised when I was told: “On average, your frost-free growing season starts May 4 and ends Oct 3, totalling [sic] 152 days.”


May 4? Who are they kidding? It could easily snow on May 4. (The above photo was taken on May 7, but I’m cheating—that’s actually hail!) The official average last frost date for Colorado Springs (downtown at 6,035 feet, slightly lower than the airport) is May 15. And since most of our city is higher than downtown, we are told to add one day for every additional 100 feet in elevation. Therefore, my average last frost date, at 7,100 feet, is May 22.

lettuce-seedlings_lah_9883As I scrolled down, I was horrified by their advice. Sow the seeds of peas (sugar snap and English) around March 5. “If the ground is still frozen, then plant these as soon as the ground thaws.” Plant tomatoes outside in early May. Sow beans, corn, squashes, cucumbers, etc. around May 4, “or if your soil is still very cold, once the soil is near 60° F in temperature.”

I guarantee my soil will still be very cold at that point. And even if May 4 is accurate, it is an average. Setting such tender, heat-loving plants out then will ensure that I lose them at times! (A more helpful statistic would be the mean last frost—the point at which my chances  of more frosts are truly 50-50.)

2009-04-12-easter-morning-snow-004The problem with this and other nation-wide gardening advice is that we don’t live in an average place. We live in a city that spans several thousand feet in elevation, in a place that can be 75 degrees at 10 in the morning, and 23 and snowing by 2:30 that afternoon. (Yes, this really happened—in late May.)

Some homes in town have sheltered yards; most of us endure gale force winds on a regular basis. Because our house is located on a hillside, cold air drains off and collects in the little dip at the bottom—I often see frost in those yards when my yard is frost-free. Our southern exposure means that our backyard thaws a couple of weeks before our front yard!

If temperatures vary that much on a small city lot, imagine how much conditions vary from one end of town to the other. Unless the app is actually specific to all five digits of our zip code, it’s not going to apply to everyone.

But how serious is this? Why not plant the cool season crops early? Peas may not sprout in early March, but won’t they come up when they’re ready? Frost doesn’t kill broccoli and cabbage. Why not set them out and hope for an early harvest?

The problem is that exposure to cold can permanent stunt plants, even if they don’t freeze and die. More than a few days in the cold will cause cole crops (cabbage, cauliflower, etc.) to bloom as seedlings, and your harvest will fit in a teaspoon. Peas may not sprout, but they can rot in cold, wet soil—or get eaten by squirrels, magpies, or other critters. Most veggie crops won’t put on any growth in cold soil. All you’re doing is exposing them to the dangers of hail, sleet, hungry rabbits, and all the other perils of being planted in Colorado.

Tomato plant after June 9 frost.

My strategy is to wait until the weather is warm and mostly settled—and then wait three more days. I’ve learned—frequently a nice, warm day precedes a cold front (we call it “the warm before the storm”). Don’t be fooled; Mother Nature is sneaky. Waiting a few extra days will pay off in faster growth, and you won’t be at the garden center trying to buy the last, pathetic tomato transplants to replace those you lost.

(Of course, I can still use their helpful chart—I just have to add three weeks to all their dates.)

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