Winter Sowing Calculator

Winter sowing is a way to start seeds outdoors in the coldest months, in clear containers that act as tiny greenhouses. There are no grow lights to buy and no seedlings to harden off, because the plants grow up outside from the start. The question every winter sower asks is how many containers to fill. This winter sowing calculator takes the number of plants you want, the seeds you sow in each container, your germination rate and the share of seedlings that survive transplanting, then tells you how many containers to prepare, how many seeds to sow and how much potting mix to buy. It works in plain numbers and quarts, like the other tools on the site.

Because winter sowing relies on weather, no seed counts are guaranteed. The calculator builds in the two losses you can estimate, seeds that never sprout and seedlings that do not survive the move into the garden, and rounds up so that you have a cushion.

How to use the winter sowing calculator

1. Enter the number of plants you want. This is the number of healthy plants you hope to set out in the garden, not the number of seeds.
2. Enter the seeds sown per container. Small seeds, such as many wildflowers, are often sown thickly, with 10 to 20 or more in a jug. Large seeds go in at a lower number. The packet or a seed guide often gives a suggestion.
3. Enter your germination rate. Use the percentage on the packet for fresh seed, or lower it for older seed. You can run your own test with the seed germination rate calculator.
4. Enter the survival rate after transplanting. Ninety percent is a reasonable estimate for winter-sown seedlings that are moved with care. Use a lower figure if slugs, rabbits or heat are a problem in your garden.
5. Enter the potting mix per container. A gallon jug filled 3 to 4 inches deep takes roughly 1 to 2 quarts, so the tool starts at 1.5.
6. Read the results. You get the containers to fill, the seeds to sow in total, the plants expected from each container, the potting mix needed and the number of 1 cubic foot bags to buy.

The winter sowing math

Each container produces a certain number of transplants after the two losses, and the number of containers is the plants you want divided by that.

Plants per container = seeds per container × germination % ÷ 100 × survival % ÷ 100
Containers = plants wanted ÷ plants per container, rounded up
Seeds to sow = containers × seeds per container
Potting mix (dry quarts) = containers × quarts per container
Potting mix (cubic feet) = dry quarts ÷ 25.71

A cubic foot holds about 25.7 dry quarts, which is why the calculator divides by 25.71 to convert quarts to cubic feet, the unit used on potting mix bags.

Worked example: 24 perennials for a new border

You want 24 plants and plan to sow 10 seeds in each container, with a 70% germination rate and a 90% survival rate. Each container yields 10 × 0.70 × 0.90 = 6.3 plants. That means 24 ÷ 6.3 = 3.8 containers, which rounds up to 4. You sow 4 × 10 = 40 seeds and use 4 × 1.5 = 6 quarts of mix, which is 0.23 cubic feet, so one bag is plenty.

Worked example: 100 plants from a bigger project

For a large planting of 100 plants, with 6 seeds per container, an 80% germination rate and 90% survival, each container gives 6 × 0.80 × 0.90 = 4.3 plants. You need 100 ÷ 4.3 = 23.1, so 24 containers, with 144 seeds and 36 quarts of mix. That is 1.4 cubic feet, which means two 1 cubic foot bags.

Worked example: 12 plants from tiny seed

Seed that is very small and slow, such as some native wildflowers, is often sown thickly and has a lower germination rate. For 12 plants, at 20 seeds per container, 50% germination and 80% survival, each container yields 20 × 0.50 × 0.80 = 8 plants. Two containers cover it, using 40 seeds and 3 quarts of mix, which is 0.12 cubic feet.

What winter sowing is and why it works

In winter sowing, you plant seeds in containers with drainage and ventilation holes, set them outside in winter and leave them to the weather. The containers are translucent, such as plastic milk jugs, so they let light in and keep the soil from drying out or washing away. Snow and rain come in through the holes and the open top. Cold, wet winter weather does the work of stratification, the cold, moist period that many perennials and native plants need before their seeds will germinate. When the days lengthen and warm in spring, the seeds sprout when the conditions are right for them, with no one to decide the date.

The method has three big advantages. It needs no grow lights, heat mats or windowsill space. The seedlings are already used to outdoor conditions by the time you plant them, so they usually need little or no hardening off. And the containers are free, since jugs, bottles and deli tubs otherwise go in the recycling.

Which seeds suit winter sowing

Winter sowing is not for everything. It works best for plants that tolerate cold or need it to germinate.

• Hardy perennials and native wildflowers: plants such as milkweed, coneflower, black-eyed Susan, columbine and yarrow commonly need cold stratification, and winter sowing provides it without a refrigerator.
• Hardy and self-sowing annuals: flowers such as calendula and sweet peas are often winter sown.
• Cool-season vegetables: crops such as kale, lettuce, spinach, broccoli and cabbage handle cold well and are commonly started this way.

Tender plants, including tomatoes, peppers, eggplant, basil and zinnias, are better started indoors later or sown outdoors after the last frost. They are killed by frost, and a winter-sown container would sprout them too early or not at all. For help with indoor timing, see the seed starting calculator and the guide on when to start seeds indoors.

When to start winter sowing

Most winter sowers begin after the winter solstice, around December 21, and continue through late winter and early spring. The aim is to sow when the weather is reliably cold, so seeds stay dormant until spring and do not sprout during a midwinter thaw. In cold northern climates, many gardeners sow from late December through February. Gardeners in milder regions can sow later, and in places where winters are mild and cold spells are short, winter sowing gives seeds less cold than they need, so a few weeks in a refrigerator before sowing is a common workaround.

It is still too early to sow if you are reading this in October. Gather your containers now, since a jug stockpile takes a few weeks, and order seed in November or December. Your own frost dates help you plan. Check them with the frost dates by ZIP code tool, and look up your region with the growing zone finder.

Choosing and preparing containers

Gallon milk and water jugs are the classic choice, but many containers work. What matters is that the container is translucent, so light gets in, deep enough for 3 to 4 inches of mix, and has room for holes. Two-liter soda bottles, clear salad or deli containers with lids, and clear plastic storage tubs also work. Opaque containers, such as colored detergent jugs, block light and are not suitable.

1. Cut the container. For a jug, cut around the middle, leaving a hinge of about an inch at the handle side, so the top lifts like a lid. Do this with a sharp knife or scissors.
2. Add drainage holes. Poke or drill at least four to six holes in the bottom. Without drainage, the mix becomes waterlogged and the seeds rot.
3. Add ventilation holes. Make several holes in the lid or the top half so that rain and snow can enter and heat can escape on sunny days.
4. Label it. Write the plant name and sowing date in permanent marker on the outside, and put a second label inside, because the outside marking fades.

How to sow, step by step

1. Fill with potting mix. Use a light, well-draining mix designed for containers, not garden soil, which compacts and may carry weeds. Fill to a depth of 3 to 4 inches.
2. Moisten the mix. Water it until it is damp throughout, like a wrung-out sponge, with a little water draining out of the holes. Seeds will not germinate well in dry mix, and waterlogged mix rots them.
3. Sow the seeds. Scatter or place the seeds on the surface, following the packet for how deeply to cover them. Many tiny seeds need light and are pressed on the surface and not buried.
4. Water gently. A light misting settles the seeds in without washing them away.
5. Close the container. Fold the top down and tape the seam shut with duct tape or packing tape. Leave the cap off, since it needs to breathe.
6. Put it outside. Set the container in a spot that gets sun and rain and is not against a heated wall. Use a tray or a pallet to keep it from blowing over, and place a rock or a weight on top if your site is windy.

Caring for winter-sown seedlings in spring

Winter-sown containers need very little attention until spring, and then only a little. As the weather warms, check the containers every week or two. If the mix is dry, water it, and if it is soggy, make sure the drain holes are clear. On warm sunny days in early spring, open the lid or make extra vents, because the inside of a clear container heats up fast and can cook small seedlings. Close it again if a hard freeze is forecast, although many seedlings tolerate frost.

When seedlings have developed a few sets of true leaves and the weather is settled, you can transplant them. Water the container first so that the mix holds together, ease out the seedlings in a clump, and separate them gently. Set them in the garden at the right spacing. Winter-sown plants are used to the outdoors, so they usually need much less hardening off than seedlings grown indoors, though it still helps to shade them for a few days after planting. The hardening off schedule calculator shows how to ease indoor seedlings outdoors if you also start any inside. For spacing, use the plant spacing calculator, and to match transplant timing to your frost dates, try the safe transplant date calculator.

Winter sowing compared with indoor seed starting

Neither method is better for every crop. Winter sowing costs less, takes less space and gives tougher seedlings, and it suits plants that need cold. Indoor seed starting gives you control over timing, which is essential for tender vegetables, and gets crops going much earlier. Many gardeners do both, starting tomatoes, peppers and herbs indoors and winter sowing their perennials, flowers and hardy greens. The drawbacks of winter sowing are less control, a dependence on weather and the need to keep an eye on containers in spring.

If you want to compare costs, our seed quantity calculator shows how many seeds a garden needs, and the garden planner helps you decide which plants are worth the effort.

Common winter sowing mistakes

• Starting too early. Seeds sown in fall or a warm spell may sprout and then be killed by cold. Wait until after the solstice.
• No drainage holes. Waterlogged mix rots seeds. Make plenty of holes in the bottom.
• Using opaque containers. Seedlings need light, so choose clear or translucent containers.
• Sealing the container shut. Rain and snow need to get in, and heat needs to get out. Add ventilation holes.
• Using heavy garden soil. It packs down and may carry weeds. Use light potting mix.
• Forgetting labels. Markings on the outside fade, and many seedlings look alike. Label inside and out.
• Letting containers bake. On warm spring days, open the lid or add vents so seedlings are not cooked.

Frequently asked questions