Monday, January 17, 2011

Winter Sowing Tutorial

 Perfect weather for gardening, right?

Here's my promised tutorial for how to winter sow.  Like I said in my last post, go to for all the details and more helpful hints from Trudi Davidoff, who pioneered the method.  Winter sowing simulates "cold stratification," where seeds are refrigerated and rehydrated before planting to trick them into believing they've been through a winter or cool rainy season so they'll germinate.

I prefer to use recycled milk jugs as my primary container.  Over the years I've experimented with all sorts of containers - I've found 2L soda bottles also work well for taller plants (like cosmos, sunflowers, or vigorous tomatoes), and I've used semi-transparent cat litter jugs to start large seedlings (daylilies in particular).  Some people swear by using gallon plastic zip-lock bags, some use recycled plastic cake containers or covered foil lasagna pans, some make pots using recycled newspaper and put those pots in plastic under-bed storage boxes or plastic bags to create the "greenhouse effect."   I've seen people use paper milk and juice cartons, and tape heavy plastic wrap over the tops and cut ventilation slits in it.  Jokes about "dumpster diving" are rampant among winter sowers, who have been known to raid their neighbor's recycling bins for suitable containers to plant in.  (I've asked neighbors before I've taken recyclables - courtesy is just common sense.)

There are a few fundamentals when you pick a container:  it must be able to hold at least 2" of soil; 3" is even better, especially for plants that will develop an extensive root system (like daylilies or tomatoes, for instance).   It must provide at least 4" of head room for the seedlings above the soil, more if you expect taller plants like cosmos or tomatoes to thrive.  It must have drainage holes in the bottom (you add those), and it must have ventilation at the top.

I start stockpiling milk jugs in the fall, rinsing them and letting them air dry in garbage bags hung in my garage.  Discard and recycle the caps, you won't need them.  (Last year, a friend of mine brought me a HUGE bag of milk jugs from her pre-K school.  I was thrilled!)  When I'm ready to start planting, I buy some potting soil.  (Regular Miracle Gro works fine.  ProMix is great, but I haven't found it in my area since we moved.  I wasn't impressed with the Organic Miracle Gro at all; the germination was very poor for me in the containers I used with it last year.  Cheap potting soil is worthless - it will turn into a brick and your seedlings won't like it.)  I keep the potting soil indoors, so I'm not trying to thaw a frozen brick of dirt in midwinter before I can use it.  I moisten the soil by pouring some warm water into it, gently, with the bag set on a towel to catch any drips.  The soil doesn't need to be soaking, just damp enough to not puff into a cloud of choking dust when you dig into it.  (Yes, that's the voice of experience here [cough, wheeze, hack. . . .].)

First I cut drainage holes in the bottom of the jug, usually four snips on the soft corners.  Then I cut the jugs around the middle, leaving room for 3" of soil in the bottom, and a tab of connected plastic between the top and bottom right under the handle.  The jug will open like a clam shell.

I add the soil to about 3" depth and gently firm it.  Then I sow the seeds.  Larger seeds I plant individually, usually 4-6 per container.  Some seeds need to be buried deeper than others - refer to the seed packet for guidelines.  When in doubt, 1/8"-1/4" is a good general depth.  Tiny seeds can be sprinkled thinly by hand, and left on the surface or very lightly covered with soil.  Some plants fare well when sown thickly, like alyssum.  The seedlings grow thickly and close together, and when you're ready to plant them out, you don't bother to separate each individual one - you simply break a hunk of soil off with its seedlings and plant them that way.  Winter sowers call that the HOS (hunk o' seedlings) planting method.  The strongest flourish, the weakest wither away, and you've still got a nice thick clump of plants.

Now here's the important part:  LABEL YOUR CONTAINERS.  You'd think this would be easy to remember, but in the thick of planting, it's easy to miss a couple, and be left with a some jugs of "mystery seeds."  Playing "name that seedling" in May isn't as much fun as you'd think, unless you like the element of surprise, lol.  Seriously, I've learned the hard way that labeling is crucial.  One thing I recommend is buying a good, weatherproof garden marker.  Trust me, Sharpies won't last on the surface of the jugs - they will fade out after a few months outdoors, and you'll be back to playing "name that seedling," or Wheel of Fortune with the remaining letters.  The garden marker is completely worth buying, and will last for several seasons of labeling.  The other thing I do is place an individual label inside every container, using a piece of recycled mini blind.  There are two reasons I use mini blinds - the plastic is weatherproof, and it's recycled (i.e. FREE).  You can write on the mini blind piece with the marker, or with an ordinary #2 pencil.  Funny enough, the combination of pencil on mini blind is the most weather-proof marking method I've found yet for labeling plants in the garden.  So, label the top and bottom half of the jug with the marker, and place a label inside just in case the outside markings don't last or are discarded when you remove the lid (which you'll do later in the season as the seedlings grow).

So, you've cut and filled your container, sown your seeds, and labeled everything.  Now you have to fasten the container closed.  I've found that regular clear packing tape works well for this purpose.  Cheap packing tape  sometimes doesn't last, but good quality Scotch or 3M brand tape is fine.  Duct/duck tape may not last (it's not weatherproof).  By spring most of it will have come loose, which makes it hard to handle the containers without spilling them. 

Now you need to make sure the soil in the containers is moist.  Bottom watering is the easiest way to do this.  Put about an inch of cool water in a sink or tub, and set your containers in it for a little while.  The soil inside will wick up the moisture through the holes cut in the bottom of the jug.  You can also dampen the surface of the soil with a sink sprayer or spray bottle.  You don't want the soil soggy.  This is just to give the seeds a little moisture to get started.  As the jugs sit outside in all kinds of weather, they'll get plenty of water. 

Now the fun part:  put the jugs outside in a location that gets at least 6 hours of sun every day, and ignore them until early spring.  I keep my containers on my deck.  At our old house they were on the stone patio.  The important thing is that they get enough sun.  It's okay if they get covered with snow - that insulates them on the coldest days, and provides moisture as it melts. 

The soil in the containers will start to thaw as the weather warms.  Condensation on the inside of the jugs is a good sign that the soil is moist enough for the seeds to sprout.  If the weather is warm and you see no condensation, you will know you need to water the containers.  It's especially important to make sure the containers don't dry out once the seedlings appear - nothing will kill a seedling quicker than lack of water.  Once the weather is consistently above freezing at night and getting into the 60s during the day, you should cut additional holes in the lids for extra ventilation.  You'll be amazed how warm those little mini-greenhouses can get, and you don't want to cook your seedlings.  When the weather is consistently warm (above 50F or so at night and into the 70s or higher during the day) you can remove the lids of the containers entirely.  This gradual process will allow the seedlings to harden off, or become accustomed to the daily temperature swing between cool nights and warm days.  You can plant the seedlings out after the danger of frost has passed.  (Some seedlings don't mind the frost, like snapdragons - know your plants, and you'll know which ones are more tolerant of cold and can be planted out earlier.)

I live in U.S.D.A. Climate Zone 5a/borderline 4b, which means we can get down to -20F on the coldest nights in January/February, even before accounting for wind chill.   I start winter sowing in January, planting the hardy perennials and hardy annuals first.  In the northeastern U.S. you can winter sow most perennials and annuals in mid to late February, and also some of the cool-weather vegetables (like onions, cabbage, broccoli, and cauliflower).  I wait until March to winter sow tender annuals like zinnias, tomatoes, and peppers.  You should look at the information on to find out more about winter sowing in other climate zones - I don't claim any expertise for the southern, central, and western/southwestern states. 

By early May your containers should look like this:

Give it a try - and have fun!
Winter sowing is a great project for schools and home educators, and also for Girl Scouts and Boy Scouts of all ages.


  1. I don't know, that "plant" in the last picture looks familiar. Just saying.

    1. LOL, Ron - just tomatoes! I think I might be sowing some tomatoes today. They're pretty tolerant of early sowing, even though they're tender as sprouts.