Choosing Containers for Winter Sowing

Winter sowing containers need to survive months outdoors through rain, freezing, thawing, sun, and neglect. Below is a practical guide to what works best and why some common suggestions can cause problems.

The Best Containers for Winter Sowing

Rigid Nursery Pots, Trays, and Flats (Plastic)

These are the containers professional growers use, for many good reasons. They:

• Hold their shape when wet and frozen

• Drain consistently

• Allow oxygen to reach roots

• Can be reused for many years

• Are easy to move and organize

Polypropylene (PP / #5) plastic is a durable nursery plastic reused for many seasons. It’s flexible but tough, with good UV and cold resistance. 

Plastic Milk Jugs & Bottles (with caveats)

Milk jugs work because they’re:

• Rigid

• Translucent

• Easy to vent and drain

Most milk jugs are made with High-Density Polyethylene (HDPE / #2), plastic. They are sturdy and handle freeze-thaw well. Some other plastics become brittle in the sun and cold, crack during freeze–thaw cycles, or shed plastic over time.

If you use milk jugs:

• Drill proper drainage holes

• Vent the top

• Expect a shorter lifespan than nursery pots

Containers to Avoid for Winter Sowing in PNW

Fiber Pots, Peat Pots, Cardboard, Paper Containers

These containers are designed for short-term indoor use, not months outside. In winter conditions they tend to:

• Wick and hold too much moisture

• Stay cold and saturated

• Lose structural strength

• Grow mold and algae

• Collapse before seedlings are ready

Metal Containers (Aluminum or Galvanized Steel)

Metal containers are durable, but they come with drawbacks:

• Limited drainage options

• Rapid heat transfer

• Deformation (especially thin aluminum)

• Potential for leaching

They work well as holders for pots or trays, but are less ideal as direct planting containers for winter sowing.

Ziploc Bags and Flexible Plastic Bags

Winter sowing in plastic bags is cheap and compact, but introduces several problems. Flexible bags:

• Pool water instead of draining

• Trap humid air (high fungal pressure)

• Shift soil and seed depth during freeze–thaw cycles

• Encourage tangled, fragile root systems

They can work for small experiments and in drier, milder climates, but they’re hard to manage, hard to scale, and easy to fail, especially for slow-germinating seeds or native plants.

Plastic Types to Avoid for Winter Sowing

Not all plastic behaves the same outdoors. Cold, sun, and moisture expose weaknesses fast.

Polystyrene (PS / #6)

Common forms:

• Foam cups and trays

• Cheap seed-starting inserts

• Takeout containers

Why to avoid it:

• Becomes brittle in cold

• Cracks and shatters during freeze–thaw cycles

• Breaks into small fragments (microplastics)

• Very short outdoor lifespan

Thin PET / PETE (#1)

Common forms:

• Water bottles

• Soda bottles

• Clamshell food packaging

Why to avoid it:

• Designed for single use

• UV exposure makes it cloudy and brittle

• Cracks along folds and seams

• Short-lived outdoors

Thick PET containers (such as reusable 5-gallon water jugs) are serviceable with modification, but not ideal. While far more durable than thin disposable bottles, PET plastics still degrade under UV exposure and require careful drainage and ventilation to avoid waterlogging and overheating. They can work for winter sowing, especially for larger batches, but generally have a shorter outdoor lifespan than nursery-grade pots made from polypropylene or HDPE.

PVC (#3)

Common forms:

• Vinyl containers

• Some clear rigid plastics

Why to avoid it:

• Degrades under UV

• Can leach plasticizers

• Not intended for soil contact long-term

• Rarely recyclable locally

Bioplastics / Compostable Plastics

Common forms:

• “Eco” seed trays

• Compostable food packaging

Why to avoid it:

• Break down prematurely when wet and cold

• Lose structure before seedlings are ready

• Often marketed beyond their real-world performance

If it’s meant to decompose, winter will help it along. 

Unmarked or Mystery Plastic

Why to avoid it:

• Unknown UV resistance

• Unknown cold tolerance

• Often lowest-quality plastic available

A Simple Rule of Thumb

The best containers:

• Stay rigid when wet and frozen

• Drain freely but don’t dry instantly

• Allow air exchange

• Survive sun without breaking down

• Can be reused year after year