Get Growing: Winter sowing jumpstarts your garden

Gardeners have used “season-extenders” for centuries, whether its digging underground storage for potatoes, building a cold frame, using row covers or, the ultimate, adding a greenhouse to your growing arsenal. But there’s one you may not know about that’s a game-changer for urban gardeners — it’s called winter sowing.

Winter sowing is a close cousin to these measures, but is a lot easier, affordable, portable and temporary — requiring no permanent construction to your garden. If you are running out of indoor space for seed-starting and grow lights — or don’t want to bother with all that — it’s a great solution. Those seedlings start small, but once they graduate into larger pots, they soon eat up a lot more counterspace.

What is winter sowing? Basically, it’s creating mini greenhouses outdoors, usually out of reused materials like milk jugs, clear liter bottles, salad clamshells and take-out trays. Plants grown outside are tougher, better acclimated to wind than those babied inside.

You can choose any vessel if it can hold about 3 to 4 inches of potting or seed-starting mix, has drainage holes, allows light and water in, but covers the seedlings from wind.

Milk jugs are the most common set-up. The iconic symbols of winter sowing — they are translucent, readily available and so easy to carry with that handle.

Here’s how you convert a milk container: Poke drainage holes on the bottom and the sides up to 3 inches (a screwdriver, awl or ice pick works well). At about 4 inches from the base, cut horizontally almost all the way through, leaving the side by the handle intact, so you’ve made a Pac-Man-style mouth to access inside for planting. Add planting medium, seeds, plant tag, write the plant name and date on the outside, and tape up that cut with duct or packing tape. Remove the milk jug cap to allow water in.

Remarkably, during my first go-round with winter sowing last year, my seedlings survived for months despite me not knowing about that last step. I guess rainwater seeped up through capillary action from the drainage holes.

We have gardener Trudi Davidoff to thank for this idea, who told the world about it on her web site Winter Sown, with the term eventually earning inclusion the USDA’s National Agricultural Thesaurus in 2006.

You can’t sow just any seeds this way, however. A thin layer of plastic doesn’t afford the same protection as glass windows or polycarbonate, after all. Seeds that can take or even prefer a little cold stratification, like cold hardy annuals, “cool season” vegetables and many perennials are perfect choices.

You can sow them anytime from fall to winter, but keep in mind the idea is to transplant your seedlings at about 1 inch high, so plan accordingly.

I just planted milk jugs of four types of (presoaked) sweet peas the first week of March.

As part of my campaign to make my lawn earn its keep, I am adding one jug each of Bellis perennis — the little white daisies you see in Seattle park lawns and playfields from spring through summer — and blue-flowered Nemophila menziesii (baby-blue eyes) to beautify our lawn while attracting pollinators. The first leg of that campaign was my pollinator bulb lawn corner started in fall, filled with crocus, muscari and Dutch iris — which I’m delighted to report are coming up now!

Clamshells with red romaine lettuce seeds and purple bok choy also went outside to jumpstart my home-grown veggie eating.

As a side experiment with the sweet peas, I also planted some indoors on a heat mat, so we’ll see which blooms first.

In case anyone is wondering, I also planted my tomatoes indoors at the same time. Winter-sown tomatoes may need extra protection in frosty weather, especially once sprouted. Anecdotal reports say that they fruit earlier and are overall more robust than those started indoor.

I recently learned of a container that seems even better than milk jugs — zip-top bags. They’re lighter, take up less space and need a lot less preparation and fussing. Just fold in three, cut three tiny corners for drainage holes, plant up and add a clothespin at the top to keep the bag open. Some plant many bags and hang them on a dowel (cutting a hole for the dowel), but I’m just going to put them in a tray or bin and check periodically that they’re still upright.

You can sow as thickly as you like, using the “hunk” sowing method, where you slice up chunks, or hunks, if you prefer, to transplant. If necessary, it’s always possible to trim a few stems to the ground to let the others thrive.

The next round will be calendula, marigolds and zinnia. I may try poppies and sunflowers, but they resent transplanting, so I’d have to be very gentle making the move to in-ground. Here goes — I’m just delighted to be planting again!