How to Winter Sow Brassicas in 7 Easy Steps

Winter sowing is perfect for many plants, but brassicas are a great way to start. Not only do I get stronger, healthier seedlings ready to be directly transplanted in the garden, I also avoid the expense and time associated with setting up grow lights and warming mats.

The brassica family includes hearty veggies like kale, broccoli, cauliflower, mustard greens, bok choy, and cabbages, among others. These delicious veggies benefit from exposure to cold temperatures, which makes them sweeter.

Planting them too late in the growing season and exposing them to hot weather may even cause them to bolt, ruining all of your hard work. While they need some cold, you can’t just direct seed brassicas out on the snow.

Enter winter sowing. With this process, you create a mini-greenhouse for your seeds using a milk jug (or another plastic container with drainage), keeping them just warm enough to shelter them from extreme temperatures. Once the weather is warm enough for germination, they will sprout, and you can open the tops of containers on sunny days and close them back up for cold late-winter nights.

So if you’re itching to get a start on your spring garden now, give winter sowing brassicas a try. It’s simple, yields great results, and uses materials you might already have around the house. Here’s a step-by-step guide to get you started!

What is Winter Sowing?

Winter Sowing is an easy method of seed-starting perfect for gardeners in cold climates. With the help of homemade mini-greenhouses, seeds germinate outdoors, sprouting when temperatures are warm enough, just as in nature! As a result, you have hardy plants ready to thrive in your garden, while saving yourself the time and expense needed to grow indoor seedlings.

Benefits of Winter Sowing Brassicas

 Brassicas are an excellent option for winter sowing. They are cold-weather crops that prefer the milder temperatures of fall and spring and have a tastier, sweeter taste when exposed to the cold.

When winter-sown, brassicas benefit from the protection of a mini-greenhouse that shields them from frost damage, allowing you to plant much earlier than you could with direct sowing.

So why not just start them indoors? I don’t know about you, but as a beginning, impatient gardener, I tended to start my indoor seeds way too early.

Not wanting to invest in grow lights and lacking in space, I had a multitude of issues: weak seedlings from lack of light, plants that needed repotting more quickly than I’d planned, and kitchen counters covered in plants. Worse, I didn’t really have the time to cart them in and out of the house to properly harden them off in spring.

Winter sowing solves all of these issues. You can set your brassicas up for success in January, February, and even March. You can then basically forget about them until spring! If you have limited space or just can’t wait to get started, this method is perfect for you. Here are some more benefits of winter sowing.

When’s The Best Time to Start?

When I grow indoors, I have to carefully plan when I begin. Otherwise, I get seedlings that are ready for transplanting in the garden far before my weather has warmed up.

I end up tracking back and forth from my kitchen, carrying my seedlings outside to get direct sun during the day, and then back inside in the evening to protect them from cold early-spring nights.

When winter sowing, timing is flexible and low-stress. Put your seeds out in winter, and they’ll naturally germinate when the weather is warm enough. They can stay in their miniature greenhouses until you’re ready to transplant.

While your specific climate will affect the growth rate of your seedlings, you don’t have to worry about making sure everything is perfect. Seeds can freeze or even sprout a bit early without damage to the plants. The milk jug (or other container) will protect them from fluctuating temperatures.

 The goal in winter sowing is to replicate the natural growing environment while keeping seedlings protected until they’re ready to transplant.

What You Need

A major perk of winter sowing is that it uses recyclable materials you likely already have, saving you money! Many types of containers will work, so choose what you already have or can access easily. Use regular potting mix, and let nature do the rest. Here’s what you need:

Getting Started

Once you’ve got your brassica seeds and collected the basics you need to begin, you can start the sowing process. Here’s a step-by-step guide:

Step 1: Prepare Your Containers

The most popular option for winter sowing is clear milk jugs with the caps left off for ventilation. Gallon-size plastic Ziploc bags, where the zipper tops let you adjust as needed, are another common choice. You can also use red solo cups in a clear storage container.

If you don’t have any of these lying around (ask local coffee shops for milk jugs!), you can use any plastic container that meets the following requirements:

1. Whatever you choose should be clear (or only slightly opaque) for sunlight to penetrate.
2. Containers need to provide enough room for root growth.
3. Choose those that fit a minimum of 3-4 inches of soil, with room for seedlings to grow above it.
4. Winter sowing containers need holes for drainage and ventilation.
5. You can easily cut or drill these yourself.
6. Tops need to be removable (you can cut soda bottles or milk jugs to make a lid).
7. This will help ensure seedlings can breathe on sunny and warm late winter days.

2-liter soda bottles, plastic take-home restaurant containers with lids will also work great. To keep things simple, I’ll describe the process for both ziploc bags and using milk jugs here:

Step 2: Fill with Soil and Plant your Seeds

Start by filling your chosen container with 3-4 inches of potting soil. When starting indoors, a sterile, soil-less seed starting mix is recommended. In winter sowing, you’re trying to mimic natural outdoor conditions as much as possible, so regular potting soil is just fine!

Now, thoroughly moisten the soil. Avoid getting it soggy or soaking wet. You want soil that’s just moist enough to stick to your finger.

You’re ready to plant your brassica seeds! Depending on your preference, you can plant densely and then thin out seedlings later on or use only 6-8 per container. 

If you like to play by the rules, seed packets give recommendations on appropriate depth and spacing for each variety. Gently tamp down the seeds, then cover them with a bit of soil.

Step 3: Secure the Container

For a while, seeds need to be protected from frost. If you’re using milk jugs, start by closing the lid, and wrap the horizontal opening closed with duct tape. You can remove the tape and open the lid when the weather is warm enough to let seedlings get some extra sunshine and room to breathe.

If you planted in plastic bags, you’ll need to keep them upright and slightly open to let precipitation in and prevent plants from getting smashed. Use a dowel or long stick, pierce a hole through several bags near the top, and rest the dowel on the edge of a plastic tub (with drainage!) or milk cart.

This way, the bags won’t get floppy or squished. Leave the zipper top open just a few inches to let the rain and snow in. Use a clothespin to keep it open if needed.

Last, make sure your containers get labeled. Make sure whatever you use can withstand exposure to the elements. I like writing directly on my containers with a sharpie or paint pen, but some prefer to use plastic labels and drop them inside.

Step 4: Pick a Location

Place the containers where they’ll get exposure to the winter elements and plenty of sun. You want robust, tough seedlings- so let the rain, snow, and wind do their work. 

If you live in a windy spot, they might get knocked over. Avoid this by placing them inside milk crates or aluminum trays rather than directly on the ground.

Step 6: Monitor Your Seedlings

A lot of winter sowers set up their containers and forget about them until spring. While this is fine, remember that they need to stay moist. If you haven’t had much snow or rain, check the soil and water as needed. 

Check for sprouts in late winter or early spring. If seedlings start to outgrow the container, leave the lids open when the weather is warm and sunny, making sure to cover them up at nighttime or if you get a surprise frost

Don’t panic if seedlings are slow to germinate. The mini-greenhouses will provide them with the protection they need to survive, but the seeds won’t sprout until outside temperatures are sufficiently warm. If you don’t see sprouts until early spring, this is normal for your climate.

The hardiness and strength of winter-sown seedlings will help them transition more easily into the garden, and often results in plants that outperform those grown with traditional indoor methods.

Step 7: Transplant

When all danger of frost has passed, you’re ready to transplant your brassica seedlings into the garden! While they can stand up to a light frost, most brassicas prefer temperatures between 65-70 degrees.

Depending on how thickly you’ve seeded, you may need to thin them out a bit. I’ve had luck with planting leafy greens like arugula in one big hunk, but for larger varieties like broccoli or cauliflower, it’s best to gently tease their roots apart, and space them in the garden with around 20 inches between each plant. 

Once planted, you can treat your brassicas just like you would if you purchased nursery starts or grew them indoors. Work in a bit of compost, water regularly, and enjoy your yummy, healthy reward!