Winter Sowing: Container Choices
By Beverley Wagar, Volunteer, Royal Botanical Gardens
Choosing the right container is probably the most important determinant of winter sowing success. So I’ve brought together what I’ve learned from many years experience with many types of containers. At the end of this article is a list of criteria that will help you decide whether a container is worth trying.
There are two variations on the mini-greenhouse concept. First is what I call the all-in-one, where the part that holds the soil (germination mix) is attached to the greenhouse top. Second is the big top setup, where a large jug holds several smaller tubs or pots that contain the soil.
These are probably the easiest containers for beginners. I like the ‘hourglass’ shaped ones because they can be cut so that the top fits over the bottom, which requires less tape and is easier to pop open if you need to add water or check for germination.
You can use straight-sided bottles (ones that don’t ‘nest’)—just line up the top and bottom and join with transparent, water-resistant tape.
Here’s a gallery of container information and ideas:
Other all-in-one options
More examples of ‘all-in-one’ containers. I don’t advise using opaque tape, though. As well, additional vent holes and better labeling would help.
If you sow directly into the bottom of a large container (as this gardener did with the top left jug) you’ll end up with a lot of seedlings that you may not need. As well, four inches of seeding mix in a lot of large containers can get expensive.
Juice jugs, if the sides are rigid enough, can make good all-in-ones. Sometimes these have large dimples on the bottom. These can be cut out and the bottom replaced with a similarly-sized piece of plastic. Or you can use it as is.
For efficient use of seeding mix and space, consider using large jugs to hold several smaller containers. I call this a ‘big top’ setup.
‘Big Top’ setups
A ‘big top’ is simply a large jug that holds several smaller containers. The jug is the greenhouse; the containers are the pots that hold the soil. Pots do not need lids or tops.
Look for large jugs that once held vinegar, kitty litter, detergent, windshield wiper fluid, etc. They should be translucent or transparent, not opaque. Cooking oil jugs can be used but it’s difficult to remove the oily film.
Big-top setups require extra work but have several advantages. They avoid the problems caused by flexible plastic. Because the sides of these large jugs are not very rigid, they bend when moved or handled. This causes gaps and cracks to open up in the germination mix and seeds sometimes get buried in the crevasses.
Another reason I don’t like planting directly into big jugs is the sheer number of seedlings they can hold. I usually don’t want a bazillion of a single species—and I don’t advise growing more than one species per container, for reasons I describe in part two of this article. Yes, I could sow more thinly and get fewer plants, but that’s a waste of expensive germination mix and also not an efficient use of the limited space on my back deck. I prefer to sow thickly in smaller containers such as deep mushroom tubs, 3.5″ pots, or 500ml yogurt tubs. Each holds a single species and they nestle securely into the big ‘greenhouse’ jugs.
A really large jug holding four smaller tubs. The two on top are deep mushroom tubs. For this one I used one edge as a hinge.
A smaller jug that nicely holds two 2″x3″ cell-inserts.
A small jug can hold a single pot.
Here’s a setup using a greenhouse tray with a high dome. Inside are either cell inserts or smaller containers.
The amount of air space above the soil level is important. This ‘head space’ prevents overheating, allows for air circulation, and permits seedlings to get quite tall before they hit the ‘roof’.
A google image search using ‘winter sow’ or ‘wintersowing’ will result in hundreds of container ideas that gardeners have tried. When deciding if something will work or not, keep the following in mind:
- Is there enough head space? Is there space for additional vent holes in the top?
- Is the top transparent or translucent?
- If the part that holds the germination mix is opaque, can it be filled to the very top? If you only partially fill a pot, the part above soil level will cast shadows on the soil surface, causing irregular germination.
- How big is the soil-holding container? You will be sowing only one species per tub, so unless you’re planting the entire back-forty, a small one is usually just fine. Remember, though, that the smaller the pot, the faster it dries out. So if you’re using really small pots (such as cell inserts or plug trays) be sure to regularly check the soil moisture levels in the spring. Do not use peat pots, which tend to keep seedlings either too soggy or too dry. They also cause trouble when pricking out seedlings since roots may be damaged if they’re attached to the peat walls.
- Can you cut or melt holes in the greenhouse material? This rules out glass and thick plexiglass.
- Can the greenhouse top withstand strong winds and the weight of big dumps of snow? I’ve experimented with setups using clear plastic bags, but I never found a reliable way to prevent them from collapsing onto the soil surface.
A variety of containers. The opaque tops are questionable but the other ones are fine.
Styrofoam cups, plastic ‘solo’ cups or any type of pot with opaque sides are good for situations where the light is directly overhead at all times, such as in an indoor setup with grow lights. Outdoors, though, the sun (in winter and spring in the northern hemisphere) is low in the sky. Opaque pots will cast a shadow over the soil surface, causing seeds near the edges to germinate later than the rest. To avoid this, fill opaque containers to the very top and tamp well to minimize settling.
Clear or translucent plastic totes are popular. Use them only as big-tops to hold smaller containers–don’t fill them directly with mix. I find them far too large for my available space. Don’t forget to make drainage and vent holes.
This gardener has used a large variety of all-in-one containers. I suspect that some are quite shallow (less than 3″ of soil mix) and most have minimal head space for ventilation and seedling growth. All seem to have vent holes.
This gardener may have trouble. The soil mix depth is too shallow. Those outside labels won’t survive long outside. Are there drainage holes in this? I recommend not mixing several species in a tub for reasons described in part two.
All containers can be saved, cleaned, and re-used next year.
Whatever containers you choose, keep records of what worked well and if you try something new, do a backup jug, just in case.
All photos, unless otherwise noted, are © Beverley Wagar under Creative Commons licence CC BY-NC-ND 4.0
About Beverley Wagar
Bev Wagar caught the winter sowing bug in 2005. She has 23 years experience making, tending, and learning about gardens. Since 2012 she’s been a volunteer at the Royal Botanical Gardens where she leads a volunteer gardening crew at the Helen M. Kippax Native Plant Garden.