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Plain as Dirt: How to elevate the quality of your seedlings

Tom Glass
Tom Glass
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As have most of us, you likely arrived here from somewhere else. And if that condition fits, you might be under the impression that seeds should be sown inside about the middle of March in time for transplanting outside in early May.

Follow that program up here without a good cold-frame or an obsessively tended network of “Wonderwalls”, and you’ll reap a harvest of only frustration.

There are no mysteries to success here, though. Let’s start by getting the timing right.



If you persist in pulling into the Vail parking garage at a quarter ’til 3 in the afternoon, then you probably aren’t aware that we are mostly frost-free after June 15. The seeds to better living are sown right there in that sentence alone.

Since most, not all, seedlings are ready for transplanting six weeks after sowing, you need to plan on sowing seed around the middle of April. Plant too early, and you will be hard-pressed to keep your seedlings compact and not floppy. Plant too late, and your harvest may arrive in our short growing season along with the first frosts.



At 8,000 feet, timing is most particularly everything. Outside of that, the rest is dependent on the color of your thumb. I can fix the timing right here in this column. However, the color of your thumb may require more intensive work, or it could be an unchangeable condition.

I’m going to get bold here and take on an intractable problem or two.

Most everyone begins growing from seed by germinating them at temperatures too warm. Compounding that problem, most everyone continues pouring on the heat until seedlings are ready to transplant. The resulting seedlings are weak and stretched.



Take, for instance, herbs: Almost all herbs germinate best at around room temperatures of 68 to 72 degrees. Tomatoes and peppers do well at the high end of that, at around 70 to 74 degrees. Bedding plant flowers can be very specific in their temperature requirements, but, with the exception of begonias, geraniums and impatiens, most do fine when germinated at room temperatures.

Years ago, I was called to the home of a friend who was deeply obsessed with growing transplants from seed. Despite his devotion, every spring he would set out the most miserable little excuses for plants. They were lanky and spindly ” the majority flopping over and breaking or rotting off at the soil line before or within a few days of transplant.

Upon learning that I had been a plug-seedling grower for a number of years, he called upon me to fix his luck. In his basement, he had constructed an elaborate grow room by covering a pingpong table with plastic that draped to the floor, trapping the heat from a small electric space heater that had been placed under the table to provide bottom heat to the table surface. Hanging about 18 inches from the top of his germinating trays that rested on the pingpong table were two fluorescent light fixtures outfitted with the best grow lights money can buy.

The temperature at plant height had to be around 80 degrees or more. I shut off the space heater and raised the lights to 4 feet above the table top. He thought I was nuts. He knew his seeds and seedlings needed more love than that. It was a false impression.

The result of our changes: His seedlings toned up. His germination percent increased. The evenness of emerging seedlings became more uniform, and therefore, his crops grew more uniformly. And, most importantly, the incidence of breaking and rotting off dropped way down.

Like spoiling kids, too much love is not productive when growing seedlings. The start of growing a toned seedling begins as soon as the tip of the radicle, the initial root tip, cracks from the seed and ” this is important -” begins to turn downward toward the soil. At this stage, drop the temperature on seedlings to around 68 degrees. This will slow the seedling emergence way down, and you will also eliminate that first stretch that ruins most seedling crops.

Then, once the first true leaves appear ” not the baby leaves called cotyledons, but the first real leaves produced as new growth ” then drop the temperature again to 60 to 65 degrees. Your plants will thank you by surviving transplanting outside into our cool nights. The benefits of growing them cooler at this stage are enormous. The plants won’t stretch. They won’t rot or break off as frequently. And they will grow slowly until the weather turns favorable outside for transplanting.

Growing seedlings cooler probably runs counter to your intuition, but you’ll grow better seedlings suited to transplanting outside ” regardless of the altitude of your garden or the elevation of your upbringing.

Tom Glass writes a weekly gardening column for the Vail Daily. Copyright 2009 by Tom Glass. E-mail comments to tom@horticulturelogic.com.


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