Seed-Starting 101: Starting Seeds Under Protection | Civil Eats

Seed-Starting 101: Starting Seeds Under Protection

This is part 2 of a six-part series on seed starting. Part 1 can be read here.

Starting seeds early, when done right, is one of the most satisfying aspects of gardening. To see young, green shoots perk up through the soil while winter carries on outside is incredibly gratifying. It’s as if spring begins as soon as the first cotyledons (first leaves) pop open. It’s also an essential part of growing tomatoes, peppers, eggplant, and other crops, which otherwise don’t have a long enough season in northern climates to mature much ripe fruit.

For the home gardener lacking a heated greenhouse, there are two main ways to start seeds under protection: indoors or in a cold frame. We’ll take a look at both strategies.


Our friend Kerry Trueman demonstrates her own indoor seed-starting technique–with decopage!–in this great video:

(Check out for more New York-based food and gardening content.)

For many gardeners, starting seeds indoors is the preferred, tried-and-true method. However, despite what most people believe, to be successful requires more than just a sunny windowsill. Successful indoor seed-starting requires the following components.

  • WARMTH. Most seeds will germinate within a fairly wide range of different temperatures. However, the swiftest germination takes place for most seeds of annual crops when soil temperatures are in the 70-80 degree range. The most notable exception to this is lettuce, which prefers a cooler temperature range of 60-70 degrees. Warmth is usually provided either by locating your seed-starting set-up strategically (near a woodstove or radiator, usually) or using a propagation mat, an electrical device that supplies bottom heat to the undersides of trays. In most cases an interior temperature of 60-70 degrees is not warm enough for quick germination, but seeds usually will germinate eventually (lack of supplemental heat is especially detrimental to peppers and eggplants, both of which are REALLY SLOW to germinate when left at room temperature).
  • MOISTURE. Seeds sown indoors are easy to water, but be sure to locate the seeds somewhere where you’ll be free to water liberally when needed. Watering can create drips and mess, and if you put the set-up in a pristine living room you risk being too precious about things to get done what has to get done.
  • LIGHT. For nearly all varieties (except lettuce), a sunny windowsill just doesn’t cut it. There do exist rare, due-south, full-sun, bay windows that just might cut it. But for most situations, extra light is necessary when starting seeds indoors. The most affordable way to provide this is to purchase a shop-light fluorescent fixture and suspend it within 1-2 inches of the emerging seedlings. Run it for 12-14 hours every day. And if you can set it up against a window, so much the better.
  • HARDENING OFF. Seedlings grown indoors are incredibly tender and sensitive, as they are subjected to neither the temperature swings nor breezes found outdoors. If you were to move them directly from the house to the garden, the shock would severely damage or kill them. Indoor-grown seedlings require full hardening off: a period of about 3-7 days when the seedlings are exposed in increasing doses to the natural elements. Start with a couple hours the first day, and gradually work your way up to 8 or 12 hours before transplanting them. Be sure to take on this process at the correct time for each variety.

Summary: Starting seeds indoors is convenient and accessible to all gardeners. Little time or money needed for infrastructure. Supplemental lighting is almost always necessary: don’t skip it! Seedlings grown indoors are ultra-tender and require careful hardening off.

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(Above, a photo of a homemade cold frame produced from easily obtained materials. Instructions on how to build this cold frame will be covered in this series.)

A cold frame is a simple structure placed in the garden that features structural sides (usually made of wood) and a top made of a transparent material such as clear plastic or glass. Starting seeds in a cold frame eliminates several of the difficulties of starting seeds indoors. However, it requires a small investment of time and money in the construction of the cold frame and careful attention on cold nights. Here’s a brief run down of what you need to know for successful cold-frame seed-starting.

  • WARMTH. From early March on, cold frames warm up significantly almost every day. When unvented, the interior temperature can easily top 90 degrees on a sunny day. The soil in seedling trays or soil blocks absorbs much of the solar radiation and heat, and the soil easily reaches temperatures that initiate seed germination. However, on cold nights the cold frame provides only 10-15 degrees of protection (depending on wind and the previous day’s high), so providing a bit of heat to stave off frost overnight is sometimes necessary. Sometimes throwing some old wool or polyester blankets on top can be enough; sometimes running a light bulb or Christmas lights within the box can do it. Generally some extra heat is wise if the outside temperature is predicted to drop below about 26 degrees and the frame contains frost-sensitive seedlings such as tomatoes, peppers, eggplants, basil, and certain flowers. (A cold frame with only brassicas and lettuces and greens will need no additional heat.) On sunny or warm days, venting is necessary–anything from cracking the lid to removing it entirely. Keep a thermometer handy: experiment a bit and you’ll get the hang of it.
  • MOISTURE. Seeds sown in a cold frame can be watered with abandon–no mess to worry about. Do monitor the seedlings at the end of the afternoon, as solar heat and breezes from venting can cause rapid moisture loss on a warm or sunny days.
  • LIGHT. When you use a cold frame, the  mighty sun takes care of your light requirements: no supplementation is necessary. Just be sure to place the cold frame in a spot that gets full sun exposure. (Keep in mind that leafless trees will fill out and shade the cold frame before tender seedlings can be put in the garden.)
  • HARDENING OFF. Seedlings grown from the start in a cold frame require almost no hardening off, as they are exposed to temperature swings and breezes from a young age. Maybe give them one or two days of resting in a semi-protected spot outside of the cold frame before putting them in the ground; other than that, you’re golden!

Summary: Cold frames provide an ideal environment for seed-starting. Gardeners are assured ample natural light and need not bother with much hardening off before transplanting. Cold nights are an issue: gardeners must monitor for sub-26 temps and provide additional insulation or supplemental heat on those nights if frost-tender crops are in the cold frame.

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Originally published on the Hudson Valley Seed Library blog

Doug Muller is co-founder of the Hudson Valley Seed Library. While teaching in Miami, he dreamed of getting back to his roots in upstate New York. In 2005 he returned, learning everything it takes to farm by hand and run a seed company. For more garden tips and to browse the seed catalog, visit You can also follow the seed library on Facebook and Twitter. Read more >

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  1. Ann
    This was really helpful information for me and my 5 year old son who wants to starts some seedlings to sell this spring. We have a cold frame but I learned a lot about how to properly use it from this article. Thanks!

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