Winter Sowing in Milk Jugs: How to Sow Seeds Outdoors in Winter

Winter sowing sounds like a contradiction, but it is one of the simplest and cheapest ways to raise a lot of plants. You sow seeds into old milk jugs, tape them shut and leave them outside in the cold, rain and frost. Each jug works as a miniature greenhouse: hardy seeds get the winter chill many of them need, germinate when the weather tells them to, and come up already used to outdoor conditions — no grow lights, no windowsill clutter and almost no hardening off. This guide explains how to winter sow in milk jugs, which seeds to sow from December to March, and how to adapt the method to a mild, wet British winter.

• When December–March for hardy seeds · late March–April for half-hardy annuals
• Container Clear or translucent milk jug or 4–5 L water bottle, with drainage holes
• Compost 7–10 cm of moist, free-draining multipurpose or seed compost
• Position Fully outside, open to rain and frost · not in a shed or greenhouse

How winter sowing works

Many hardy perennials, shrubs and wildflowers carry a built-in dormancy: their seed will not germinate until it has spent weeks cold and damp, the way it would on the ground over winter. In a jug outdoors, that chilling happens naturally. As days lengthen and the jug warms in the sun, the seed senses spring and germinates at the right moment for that species. The translucent plastic lets in light and keeps off wind and the worst of the rain, while the open neck lets moisture and fresh air in. Our guide to growing perennials from seed explains cold stratification in more detail.

The other benefit is that seedlings grow up in real outdoor light and temperatures. They are short, sturdy and already hardened, so they skip the leggy windowsill stage described in our guide to leggy seedlings.

What you need

• Translucent containers. Milk jugs, large water bottles or clear takeaway tubs with lids. Opaque or dark plastic blocks too much light.
• Compost. A free-draining multipurpose or seed compost. In a wet British winter, mixing in a handful of grit or perlite helps.
• Tools. A craft knife or scissors, a skewer or drill for holes, duct tape and a paint pen or pencil — ordinary marker fades in the sun.
• Seed. Hardy perennials and annuals for midwinter, cool-season vegetables from February. Fresh seed gives the best results; see how long seeds last.

How to winter sow in milk jugs: step by step

1. Cut the jug open Cut around the jug about 10–12 cm from the base, leaving a 3 cm strip uncut below the handle as a hinge.
2. Make drainage holes Push six to eight holes through the base. Throw the cap away — the open neck is the ventilation.
3. Fill and soak Add 7–10 cm of compost, water until it drains freely, then let it settle.
4. Sow at the usual depth Sow thinly, following the packet: fine seed on the surface, larger seed covered to two or three times its size.
5. Label twice and tape shut Write the name and date on the base and on a label inside, then tape the seam all the way round.
6. Put it outside and leave it Stand the jugs together in an open, sunny spot, in a crate or against a fence so they do not blow over.

Which seeds to winter sow — and when

December–January: seeds that need a cold spell. This is the time for hardy perennials with seed dormancy: delphiniums, columbines, echinacea, lavender and many native wildflowers. They need the longest chill, so sow them first. Our December and January sowing collections list more.

February: hardy annuals and early vegetables. Cornflowers, larkspur, nigella, poppies, calendula and snapdragons all cope with frost as seedlings. So do kale, cabbage and onions. See the February collection.

March: cool-season crops. Lettuce, spinach, other brassicas and sweet peas do well sown in jugs in March.

Late March–April: half-hardy annuals. Zinnias, cosmos and similar flowers can go into jugs too; they simply sit there until it is warm enough. Heat-lovers that need a long season, such as tomatoes and chillies, are better started indoors, and root crops like carrots and parsnips dislike being moved — sow those direct.

Winter sowing jugs vs pots in a cold frame

Caring for the jugs until spring

For most of the winter, do nothing. Snow and frost are not a problem — they are the point. Check now and then for condensation on the inside: if there is none on a mild day, water gently through the neck or stand the jug in a tray of water for an hour. In a wet spell, make sure water is draining out of the base.

The real danger comes in spring. On a sunny March or April day, a sealed jug can overheat and cook its seedlings. Once seedlings are up and daytime temperatures pass about 12–15 °C, open the lids during the day and close them at night if frost is forecast. When nights are reliably frost-free, remove the tops completely. Plant out in small clumps — ‘hunks’ — when seedlings have two to four true leaves, or tease them apart and pot them on. Because they have grown up outdoors, they need little or no hardening off.

Common mistakes

• Too few drainage holes. A wet British winter turns compost to mud. Make plenty of holes and use free-draining compost.
• Keeping the jugs sheltered. In a shed, greenhouse or under the eaves they miss the rain and the chill. They need to be fully outside.
• Forgetting to vent in spring. Closed jugs in bright sun overheat fast. Open them on warm days.
• Sowing too thickly. Dense clumps are hard to separate. Sow thinly, especially fine seed.
• Labels that fade. Marker pen disappears in a few weeks of sun. Use a paint pen or pencil, and label inside as well.

Frequently asked questions

Will frost kill seeds in milk jugs?
No. Hardy seeds are adapted to freezing and thawing, and for many perennials it is exactly what breaks their dormancy. Seedlings of hardy plants also tolerate frost. Only half-hardy annuals should wait until late March or April.

Does winter sowing work in a mild climate like Britain?
Yes, but drainage matters more than in snowy climates. Use grit in the compost, make extra holes and expect some hardy annuals to germinate early in mild spells — they are tough enough to cope.

When will my winter-sown seeds come up?
Most germinate between late February and April, depending on the species and the weather. Some slow perennials take until late spring. Do not throw a jug out until early summer.

Can I winter sow vegetables?
Yes, cool-season crops such as kale, cabbage, lettuce, spinach and onions do well when sown in February and March. For the full list of crops and dates, see our vegetable sowing calendar.

A row of milk jugs by the back door this winter can give you hundreds of plants by May for the cost of a few packets of seed. Pick a handful of hardy perennials to start in December, add annuals and vegetables as the days lengthen, and let the weather do the rest. Browse our annual flowers, vegetable seeds and all our seed collections, or explore the rest of our growing guides.