Winter Sowing Perfected

by Michelle Mero Riedel

After 15 years of growing plants this budget-friendly way, here are my best tips and techniques.

With winter sowing, you create mini greenhouses that can be set outside before you would normally plant seeds. The seeds germinate when the temperature is right and the greenhouses protect the seeds from our changeable weather.

To construct a mini greenhouse, you’ll need:

  • Gallon plastic milk or water jugs
  • Quality potting soil (not seed-starting mix)
  • Knife, scissors, drill or something sharp to cut the container
  • High-quality duct tape
  • An environmental marker so the ink doesn’t fade
  • Plant labels (I cut up plastic mini blinds!)
  • Seeds


GETTING STARTED

  1. With a scissors, drill or knife, cut several ½-inch holes in the container bottom. If you’re using a drill, a couple ½-inch holes will do. If you’re using a knife, make four cuts on each bottom side. It’s not unusual for the holes to get clogged or closed up, so be prepared to add more as spring progresses.
  2. With a scissors, about 4 inches up from the bottom, start at the handle and cut a horizontal line from one side of the handle to the other. Peel back the top by the handle, but keep it hinged so it’s still in one piece.
  3. Add about 3 inches of potting soil. Add water to the soil until it is saturated. Another option is to saturate soil in a bucket then add it to your mini greenhouse.
  4. Smooth the top of the soil, then add seeds. I usually overseed for perennials, annuals and herbs, and add one to two seeds per plant for vegetables. If the seeds are large, press them into soil; otherwise, just place seeds on top.
  5. Cover the seeds with about ¼ inch of soil, or about the length of the seed. Then lightly water or spritz so the seeds don’t move. It’s best to have the seed surrounded by moist soil.
  6. Add a plant label. When you remove the cover in the spring, you’ll want to know what’s growing.
  7. Line up the top of the container with the bottom and seal it with duct tape. I like to cut about two 12-inch pieces, starting the tape at one handle side and bringing it to the middle, then the other handle to the middle.
  8. Remove the milk carton top. Wind, snow and rain should reach inside the container, creating a greenhouse with a small opening.
  9. Place your greenhouses in an outdoor, sunny location.
  10. Wait patiently for germination to occur.

You're probably wondering, “Why not grow plants indoors under lights?” You certainly can, but inside sowing requires lights, trays, seed-starting soil, heating mats and shelves. It’s a rewarding but expensive process. Winter sowing is a budget-friendly alternative.

 

WINTER VS. SPRING SOWING

Winter sowing is especially successful with perennials, as their seeds need a cold snap (stratification) to germinate. You can place perennial greenhouses outside at the beginning of the year, but I prefer to wait until February, when Mother Nature dishes out more moisture. You can plant perennials until the end of March, as we still get cold weather in April.

Vegetables, annuals and herbs can also be sowed in greenhouses, but do it in the spring. I start them at the end of March through the third week of April. By the end of April and early May, the seeds can be direct sowed into the ground.

I’m especially fond of using greenhouses for my veggies, herbs and annuals. I can see what germinates, care for them with the sun’s heat, then transplant them at my leisure. Plants grown in mini greenhouses tend to be larger than those directly sowed in the ground. This is especially important for the tropical veggies like tomatoes and peppers that don’t always finish the season when fall arrives.

 

GERMINATION, WEATHER AND MOISTURE

If spring temperatures are normal to warm, perennial germination will typically occur in early to mid-April, but it depends on which perennials you plant. I have guaranteed early germination with lupine, Angelica gigas, poppies and rudbeckia. The open-top containers make it easy to spot green seedlings. Perennials are the easiest to care for in early spring. No matter the temperature, they will survive. If you’re trying this method for the first time, perennials are a solid choice.

Annuals, vegetables and herbs require more attention in the seedling stage than perennials. For example, if tomatoes and peppers are in the seedling stage and the temperature is below freezing in early May, move the greenhouses to a protected location, such as your house or garage. Then move them back into sunshine the following day.

In spring, I watch the weather carefully. I need to know the day’s high and low temperatures. If weather is hot, I may move the greenhouses into part shade, receiving only morning sun. If weather is chilly, I’ll take advantage of all-day sun and move less-hardy plants indoors overnight.

The greenhouse soil should be moist at all times, especially when seedlings are present in the spring. You’ll know the greenhouse environment is perfect when condensation clings to the container. When necessary, water with a mister or gently allow the water to run down the sides of the greenhouse. Water gently so you don’t disrupt the seeds, particularly before germination. When plants reach a mature stage and display many leaves, regular watering is acceptable.

 

DIVIDE AND TRANSPLANT

When roots are abundant, typically in early to late May, it’s time to divide your plants. I prefer to have the entire container of soil well-rooted without loose soil, but that doesn’t always happen.

For herbs, annuals and perennials, I use a mass of seedlings to formulate one plant. This one-plant-mass will be a strong survivor when I transplant it into the garden. I prefer to initially plant into a smaller pot and allow it to grow until it is root bound, usually by mid to late June.

For vegetables, one to two plants are sufficient. Veggies grow fast with strong roots, so they transplant easily. Again, I transplant them into smaller pots, allowing them to grow until my garden area is warm and ready. They grow faster in the warm soil pots than they will in the cool, spring ground. By early to mid-June, I’m ready to plant.

 

Michelle Mero Riedel is a master gardener, professional photographer and regular contributor to Northern Gardener magazine.

CHOOSING A CONTAINER

I’ve tried many types of containers for winter sowing, but I keep going back to gallon-sized milk or water jugs. Here’s why:

  • Pliable plastic. The easier the container is to cut, the safer it is for you. Trying to cut stiff plastic is dangerous. After many spring rains, you may need to add more holes, so it’s nice to effortlessly snip the bottom side of the container with scissors.
  • Height. Containers need to have enough height for 2 to 3 inches of soil, plus 6 to 8 inches of plant growth.
  • Hole. Having a hole in the top makes it easy to see what’s going on inside. In the spring, you need to monitor growth and soil moisture.

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