Winter sowing is a seed starting method that makes use of outdoor conditions (cold weather and sunshine) to germinate seeds. Grab a milk jug or other container and let’s get started.
If you would rather sow indoors, see Seed Starting for Beginners for my best tips.
Winter Sowing: Starting Seeds Outdoors
When you first learn about winter sowing it seems a bit mysterious. How can seeds germinate out in the cold—don’t they need warmth and moisture to sprout?
Yes they do. But, some seeds also need a cold chill in preparation for germination and that’s where this method can help.
We call it ‘winter sowing’ but instead of sowing the seeds directly outdoors we’re sowing in clear, closed but ventilated containers like milk jugs that are placed outdoors to experience winter conditions. The seeds remain dormant while chilling, and, as spring approaches, light and heat increase and germination is triggered.
Which seeds work best?
Winter sowing is best for any cold hardy seeds that require ‘stratification’—that cold chill I mentioned that prepares some seeds for germination. By using containers, we’re basically mimicking what nature does but giving the seeds better odds by sheltering them from extreme conditions and critters that might eat them.
The group of seeds that need a cold boost includes favorites like delphinium, milkweed, lupine, and columbine in addition to cold climate native plants and some trees, shrubs, and vines. These are temperate (not tropical) plants. I’ve included an extensive list of seeds to try (below).
Can I winter sow seeds that are not cold hardy?
Yes. This method can work for any seeds. The difference is, cold-tolerant seeds are started first in the coldest months of winter to provide the necessary chilling period.
Any tender or warmth-loving seeds are started later in spring when there is little risk of night-time temperatures dipping too low. At this point the milk jug is really just acting as a mini greenhouse.
I’ll walk you through the basic steps and supplies needed.
If you just want the list of suggested seeds to try, you can jump to it here: seeds to winter sow.
Getting Started With Winter Sowing
Winter sowing can take place from winter to spring, up until a few weeks before last frost.
It’s helpful to organize your winter sowing into three main groups based on when the seeds can be started. There is a lot of overlap between Groups A and B.
A) Early Winter Sowing
Sow seeds needing weeks or months of stratification.
This group includes many flowering perennials and cold-tolerant vegetables in addition to native plants, trees, shrubs, and vines.
For me in zone 6, sowing time is December and January.
Examples: delphinium, milkweed, lupine, columbine, clematis. There are more examples listed here.
B) Winter Sowing
Sow cold hardy perennials or annuals.
For me in zone 6, sowing time is February and early March.
Examples: Brussels sprouts, Monarda (bee balm), pansies.
C) Late Winter / Early Spring Sowing
This group includes tender annuals including flowers and vegetables that do not require stratification and need consistent warmth.
Here in zone 6, sowing time using the winter sow method is late March and April.
Examples: cosmos, zinnia, tomato, eggplant.
The example I’ve used for this article is 2-gallon milk jugs but you can use just about anything that fits the criteria. Check your recycling bin and ask friends for what you need.
Use containers (usually plastic) that:
- Allow light in
- Can withstand winter weather
- Provide adequate room for root growth and height of young plant (not an issue if used for seedlings only)
- Can be drilled for drainage holes in base and ventilation holes up top (more on this below)
- Milk, juice, or water jugs
- Large beverage bottles
- Clear clamshells / food containers (double them up if they have too many vent holes)
- Cat litter jugs
- Clear ice-cream tubs (with lids)
- Plastic beer cups (in clear tote bin with lid)
- Clear tote / bin with lid
How to Clean Containers
Wash everything in soapy water, rinse thoroughly, and then disinfect with a bleach solution (4 teaspoons bleach per quart of water for at least one minute).
Create Drainage and Vent Holes
The bottom of your containers will require enough drainage holes to easily allow excess water to flow out.
The top of the container requires ventilation. For milk jugs we achieve this by removing the lid, otherwise drill a series of small holes in the top.
For bins, we can leave the lid ajar to allow air flow.
TIP: It’s much easier to drill holes in the base from the inside of the container aiming onto a scrap piece of wood while the plastic is fully supported. Otherwise, if you drill from the underside into the empty jug, it’s more likely to crack.
Containers used for winter sowing may last just one season depending on how brittle the plastic becomes under outdoor conditions. Sturdier tubs may endure many seasons of use.
3Prepare Milk Jugs
We cut across the middle of the milk jug to create a base where the potting mix goes while keeping a top to form a mini greenhouse.
You want to allow at least 4 to 6-inches of room for potting mix and watering in the base of the jug.
These are 3 different ways to do this:
- Make a partial cut around the middle of the jug (but below the base of the handle), leaving a section that forms a hinge (as shown in diagram, above).
Draw a line where you want the cut, make pilot holes, and then cut, leaving a hinge
- Cut a little door in top half of the jug, leaving a top hinge and room to place potting mix inside. It should be big enough to fit your hand inside.
- Cut the jug into two sections (top and bottom) and cut vertical slits into the top lip of the base. This will allow the top of the jug to sit nicely over the base (no tape required).
Make Cuts and Holes
Wear safety glasses and always aim away from yourself while cutting.
Cuts and holes may be made with some combination of:
- Exacto knife
- Box cutter
- Serrated knife
- Wood burning tool
- Heated screwdriver tip
- Electric drill
Tip: It may be easiest to start with pilot holes and then cut with heavy duty scissors or a similar tool working from hole to hole.
Use organic potting mix, not seed starting mix which will dry out too quickly. You could also use soil from your garden if it’s up for the job—I don’t use mine because it’s way too sandy.
I look for a potting mix deemed safe for food crops (growing vegetables) so I don’t have to worry about where it ends up.
Pre-moisten Potting Mix
Before adding potting mix to your jug, moisten it with water. It should be moist enough to form a ball when you press it together but not so much that water drips out.
Some seeds like salad greens just form shallow roots while others (e.g. tomatoes) go deeper. A depth of at least 4-inches works for most plants for the seedling and young plant stages.
If your potting mix is too shallow, the plant roots could freeze early on or become rootbound later.
Seed Sowing Depth
Winter sowing is just like any other seed sowing: follow the instructions on your seed packet for things like sowing depth and whether the seeds should be covered. Most germinate in darkness but there are some that need light to sprout.
This comes down to personal preference. Some gardeners like to sow a lot of seeds and thin them out later, leaving only the most vigorous growers.
I have friends who sow a large number in each jug because they need to grow high volumes for plant sales—but you do have to be ready to move them to (a zillion) little pots once they are established..
When I know my seeds have good germination rates, I sow just what I need with a few extra for insurance because I do not like thinning seeds or transplanting.
Do what fits your situation: how many plants you hope to grow, how much room you have, and how much work you’re equipped to do.
One Seed Type Per Jug
Ideally we start just one type of seed in each milk jug otherwise it gets tricky with varying plant heights and needs as things grow.
An exception is something like leafy greens or mesclun mix where the different varieties grow nicely together in a compact space.
As mentioned in the Timing section, it’s helpful to start seeds in 3 different phases depending on how long of a stratification period they require (or not). This means starting batches of seeds in early winter, winter, and late winter / early spring depending on the seeds.
We stop using the winter sowing method each spring when outdoor conditions are right for direct outdoor sowing.
6Tag and Seal Jugs
Tags & Labels
I’ll remember what I sowed! says every gardener who later forgets.
Be sure to keep track of what you’re growing.
There are lots of options for tagging your sowing jugs. Most of us do several things just in case.
- Use paint marking pen or another UV-resistant marker, grease pen, or pencil. Regular Sharpie pens do not work: they fade in the sun. Markers need to be oil-based to work.
- Label the jug, write on the tape, and/or place a plastic plant tag inside the jug.
I record the type of seed, source (seed company), and date sown.
Another option is to assign a unique number to each jug and keep a separate, corresponding written record.
Seal the Jug
We need to close up the jug (or container) except for the top ventilation hole(s) so they can act like mini greenhouses, retaining heat from the sun without overheating.
To seal milk jugs shut, use duct tape, or poly tape.
Other options include punching holes along the seam and tie the top and bottom together with twist ties or shoelaces.
I’ve also used old zippers affixed to the jug using GE II Silicone sealant as adhesive.
At last, the fun can happen.
Sun is key for winter sowing, particularly the early group requiring stratification when weather is coldest and days are short.
Place your jugs in a sunny location, avoiding any puddle-prone areas, out of direct wind (which can knock the jugs over or dry out the potting mix).
This could be on the ground or on a patio table. You may also want to run twine through the handles to hold the jugs together or sit everything in tubs with drainage holes for extra security.
Snow and rain are welcome. Snow provides insulation and some rain will get in the top vent to moisten the soil.
It’s up to you to monitor the jugs to ensure they don’t overheat or dry out: there’s more on this below.
8Ongoing Care Outdoors
Water and Ventilation
Ongoing care involves ensuring the potting mix stays moist and the jugs do not either overheat or succumb to unusually cold conditions once your seedlings have appeared.
For the early seeds that require stratification, if the weather stays below freezing, there won’t be anything to do.
It’s when we get warm spells (a few degrees above freezing or more) or wild weather that you’ll want to check on things.
The first thing you’ll notice is condensation inside the jug. This is good. It means there is moisture available and the jug is indeed behaving like a greenhouse.
On warm or intensely sunny days (even if it’s cold), check the temperature in the jug and whether the potting mix needs watering.
I like to use a no-contact thermometer like this one to measure the temperature inside the jug. If things are heating up, I have no problem opening the jug right up to cool things off for any cold hardy seeds.
However, spring-sown seeds like tomatoes and peppers do not like temperature swings so they require more care. I only open up the jug to vent them if the outside air temperature is within 10 degrees of the inside jug temperature. Or, I bring them indoors for any maintenance.
Late Cold Spells
The other wildcard is unusual spring cold snaps. There is a progression as the seeds sprout and the young seedlings start to grow their mature leaves. Tolerance for cold and temperature swings varies by species.
If a rogue spring storm or freeze is coming, I cover the jugs of tender seedlings until the risk is over. You can use old blankets, tarps, straw, or place the jugs in bins in a sheltered location. Just remember to uncover them when the freeze is over.
As mentioned, never let the potting mix dry out.
You can water the jugs by sprinkling water in the open top or sitting the jugs in a tray of water, letting the potting mix take up the water through the bottom drain holes for approximately 30 minutes or until adequate moisture is obtained. Or remove the tape and water that way.
I am a lifelong moisture meter user but your finger tip (down to a knuckle) can also tell you how moist soil is.
Most seeds in winter sowing conditions take approximately 6 to 8 weeks to germinate—unless they’re in a deep freeze which pauses everything.
It’s always fun to check on the jugs and see tiny green shoots appearing inside!
Some seeds are total slow pokes so be patient. I’ve had delphiniums, milkweeds, and columbines all take their sweet time. The milkweeds in particular may not sprout until their second year—so patience can pay off.
Also, tree seeds often require a double dormancy (two winters) before they sprout.
Fertilizer is not required until there are a few sets of mature leaves on the plants. I hold off knowing there will be nutrients available once I plant everything in the garden.
9Thin Out, Grow On, or Transplant
As your seeds grow and the weather warms, it’s time to allow more ventilation either by increasing the number of vent holes, opening the jugs up each day, or removing bin lids.
Depending on conditions, you may need to close them each evening as the sun goes down and open them up again mid-morning each day.
This process is called hardening off: getting plants used to natural outdoor growing conditions.
Thinning out is generally done when a seedling has at least two sets of mature leaves: you carefully remove all but the best growers.
If your seedlings are getting crowded and it’s too soon to plant them out, you may need to transplant them to larger pots which can remain outdoors in bins (with lids ready in case it gets cold).
Use the instructions on your seed packets to know when transplanting is recommended. This is usually after the risk of frost has passed but there’s always some exceptions.
Seeds for Winter Sowing
Seeds suitable for this method can be divided into three main sowing groups based on when you sow.
A) Early Winter Sowing
Seeds needing weeks or months of stratification.
This includes many flowering perennials in addition to native plants, trees, shrubs, and vines. Some vegetables like broccoli and Brussels sprouts can also handle an early start.
For me in zone 6, this is December and January.
B) Winter Sowing
Seeds needing moderate periods of stratification and / or hardy perennials and annuals.
For me in zone 6, this is February and early March.
I’ve listed some possibilities below—there are hundreds more. Seed catalogs sometimes list suggestions by sowing time as well.
Be sure to pick seeds suited to your hardiness zone and growing conditions and avoid invasive species.
- Bee balm (Monarda)
- Bell flowers (Campanula)
- Blazing star (Liatris)
- Clematis (Group A)
- Columbine (Group A)
- Coneflower (Echinacea)
- Delphinium (Group A)
- Foxglove (Digitalis)
- Joe Pye Weed
- Milkweed (Group A)
- Lupine (Group A)
- Rose campion
- Brussels sprouts
- Leafy greens
C) Late Winter / Early Spring Sowing
Tender annuals including flowers and vegetables that do not require stratification and need consistent warmth.
Here in zone 6, this is late March and April.
- and other annual herbs
- Bok choy
- Brussels sprouts
Good luck with your winter sowing. If you’re like many gardeners, once you try it, you’ll want to experiment more.
~Melissa the Empress of Dirt ♛
How to Winter Sow Seeds in Milk Jugs
Supplies & Materials
- 1 pack Seeds cold-hardy
- Drill holes in base of clean milk jug to provide drainage.
- Cut open clean milk jug horizontally around middle leaving section below handle to form hinge. Cut should be at least 4-inches above base.
- Remove lid.
- Add 4-inches of moistened (water only) organic potting mix to base of milk jug.
- Sow seeds following depth instructions on packet.
- Label jug and add plant tag noting seed names, source, and sow date.
- Tape or tie milk jug shut around middle, keeping lid off.
- Place in sunny location outdoors away from wind or puddles.
- Check jug on sunny or warm days (above freezing) and water potting mix as needed.
- Transplant seedlings to garden following timing advice on seed packet.