Hello! For the last six years I’ve been winter sowing annual, perennial, and vegetable seeds to get a head start on gardening for the year! I’ve found it to be the most economical and simple way to produce cold-hardy seedlings that requires minimum effort. My process really works, as I’ve grown thousands of plants in this manner (no exaggeration). Read on to see my tried and true process!
What is Winter Sowing?
Winter sowing is the process by which we sow seeds outdoors in the winter, contained within mini-DIY green houses! Winter Sowing allows mother nature to cold stratify our seeds naturally outdoors. The DIY mini-green house allows our seeds to germinate while we are still getting freezing temperatures at night.
Winter Sowing Has Big Benefits
By using miniature greenhouses and winter sowing, our seedlings can survive freezing temperatures and be ‘cold’ hardy by early Spring! So, there is no ‘hardening off’ of young seedlings as you have to if started indoors. They are ready to plant in the ground directly after the last frost date.
In nature, seeds spending the winter in the ground may also serve as one of the natural ‘scarification‘ methods. As the freeze/thaw cycles will naturally crack or weaken the hard outer shell of some seeds.
- A clean milk jugs, or plastic container/pot with covered dome
- Scissors or sharp knife
- Basic potting soil
- Watering can or sprayer
Step by Step Process How to Winter Sow Seeds
Note, I generally try to have all of my seeds winter sown by New Years day. There are no ‘one-size fits all’ rules to this. However, at the end of this process I will discuss when you need to be done winter sowing your seeds in more detail.
1 – Prepare your container for winter sowing
There are many different options for a container. Everything from a milk jug or take-out food container to a specially purchased 1020 tray with plant inserts and plastic dome.
For a 1020 Plastic tray with inserts and plastic dome:
Add air holes: Use a pocket knife to add drain holes to the 1020 tray if none are present. Holes should be 3-6 mm diameter (1/8″-1/4″)
Use knife to poke holes around the side of the dome (every 4″ or 10 cm). Also, poke about 6 – 8 holes that are 1/4″ diameter (6 mm) in the top of the dome.
How to prepare a milk jug for winter sowing
Discard the lid to the milk jug. You don’t need it. The top of the jug will serve as air exchange/ventilation later in Spring.
Use knife to add drain holes about 1/8-1/4″ diameter (3-6 mm) on the bottom of the milk jug.
Stab the knife into the bottom of the jug (a Philips head screwdriver works as well). Then, twist the knife to make the hole the appropriate size. I generally add 5-8 drain holes, distributed across the bottom.
Use a knife or scissors to cut the milk jug nearly in half. Almost like making a flip-up lid on the jug.
2 – Fill your containers with moistened potting soil.
Mix potting soil or seed starting mix with water until it is damp and clumps. Then, add the soil to the plastic containers to about 1/4″-1/2″ (6-12 mm) below the top edge.
3 – Plant your seeds.
Plant seeds to the appropriate depth. Use your seed packet instructions to learn the appropriate depth.
4 – Label your seeds/containers.
I use masking tape and a grease pencil (also known as a china marker) marker to label plastic pots. For milkjugs, I just write directly on them.
The grease pencil tends to last all winter. Just keep the label pointed away from the sun.
5 – Secure the lids on your containers.
For a milk jug or something similar, just use duct tape to seal the outside. A single layer should do it. Test your jug by picking it up by the handle!
For a 1020 tray w/ dome, I use duct tape and twine to ensure the lid stays secure.
6 –Place your container outside.
I put mine in an area that receives no sun. My reason for doing this is it prevents any premature germination (I’ve had seeds germinate in early February before!).
7 – Wait until Spring!
You shouldn’t need to water your containers. The plastic domes will act as green houses and help retain moisture.
But, if the outside temperatures are above freezing, and your potting soil looks dry you may need to water.
To safely add water to your containers, do one of the following methods:
Water from the bottom. Place your milk jug container or tray in a large plastic tray or kiddie pool, and add some water. The water will enter the drain holes, and through osmosis moisten the potting soil.
Mist the soil. Carefully remove the lid and use a sprayer to mist the soil until saturated. Then secure the lid.
Once Spring arrives, put the containers in a location that receives morning sun / afternoon shade.
This step is really helpful. Allowing them morning sun but afternoon shade will get the mini-greenhouses warm, but avoid the hotter afternoon sun.
Doing this really helps the seeds germinate quickly, but reduces the overall heat load. Reducing the heat load means less watering for you, so easier!
After germination, open the lids during the daytime
This is important! – Once outdoor temperatures approach 60 F (15 C), remove the lids.
In direct sunlight and warm temperatures, the temperature inside the dome can get too hot and kill your seedlings.
I’ve had seedlings get cooked by leaving the lids on trays in direct sunlight and temperatures that were 60F (15 C).
Remove the seedlings. You can dig out the chunk of soil from the milk jug / container. I prefer to do this when seedlings have two sets of ‘true leaves’.
If you let the soil get slightly dry (but still moist), you can easily cut up the soil inside the milk jug. Just cut it up into chunks that are about 2″ by 2″.
Then, just plant your seedlings.
When should you winter sow seeds?
There are no ‘cut off’ dates to winter sowing seeds. The real determining factors are what kind of cold stratification your seeds need. In general, if you look up your typical last frost date, and subtract the number of recommended cold stratification days, you can determine when you need to have the seeds sown.
For example, many perennial flowers require a 30 or 60 day cold moist stratification period. So, for my area, USDA zone 6 that means I should have all of my seeds winter sown no later than March 1st, as this is 60 days before our historical last frost date of May 1st.
When to winter sow vegetables
For vegetables, I generally will winter sow them on April 1st. This usually includes tomatoes, several kinds of hot peppers, zucchini, and cucumbers.
What kind of containers should you use?
The kinds of containers I use for winter sowing depends on how many plants I need. If I feel that I only need six plants of a species, then I use 6-packs and the 1020 tray. If I think I want more seedlings, then I utilize a milk jug or larger plastic container.
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