Introduction: CMS Mini-Greenhouse

The seeds of many common, pollinator-friendly plants in northern North America require cold moist stratification (CMS) to germinate. Folks on the site from Michigan pioneered a device made with recycled materials to germinate milk weed seeds (milk weed is an essential plant for Monarch butterflies). I experimented making some of my own last winter to germinate purple coneflower, false sunflower, black-eyed Susan, and yarrow seeds as well as milk weed. So this is a remix and expansion of their Part 1 and Part 2 articles.

Step 1: Materials

1. You will need tall, rectangular plastic bottles. Fruit juice bottles from Rougement are perfect for this: they have horizontal grooves you can follow for cutting, their paper labels are easy to remove, they are clear, their opening is wide enough to allow in some rain but narrow enough to prevent drying out, and their rectangular form makes for compact storage. Ocean Spray bottles will also work, though their oval labels need to be pried up and lifted with a knife. You will not need the caps (so you can scrounge capless bottles).
2. Seed-free potting soil
3. Some moisture retainer such as coir or peat moss.
4. Flat part of an aluminum pan or pie plate to make labels out of.
5. Coated wire to attach labels to bottles (easily found in a dollar store).
6. Tape. The absolute best is 2" UV resistent tape from Lee Valley (also known as greenhouse repair tape). It can be cut in half; if taken off the planter/green house, it will stick back on; and it is translucent. But you can also use duct tape; it, however, cannot be reused. The tape you use needs to stand up to sun, rain, and cold for several months. You are using it to make your little greenhouse weather-tight. Yeah, regular masking tape won't cut it.
7. Seeds (not shown in photo)

Step 2: Tools

1. Trowel to mix soil and coir (or peat)
2. Utility knife to make bottle cuts
3. Tape measure
4. Scissors for cutting tape, wire, and aluminum
5. Small metal skewer for puncturing labels and writing bottle contents on them

Step 3: Prepare Bottles

1. Remove and discard cap. Rinse out bottle. Remove the label.
2. Using the utility knife cut triangular holes in each bottom corner of the bottle. The triangles should be roughly a quarter inch on each side.
2. Measure 4 inches (just over 10 cm) from the bottom of the bottle. Use the nearest ridge above that as your cutting guide.
3. Use the utility knife to cut the two wide sides and one depth side of the bottle, leaving one depth side intact to serve as a hinge.

Step 4: Prepare Soil

Mix 3 parts of potting soil with one part of coir or peat moss. The soil should be moist enough to form a clump when pressed in your hznd. Add water if necessary to achieve this.

Step 5: Put Soil in Bottle and Seed

Add soil within an inch of the cut. With these 2 liter/quart bottles, I usually seed for two seedlings with 2 seeds per area half. Put the seeds on the soil and lightly cover with soil (we're trying to mimic CMS in the wild here.)

Step 6: Seal the "greenhouse"

Cut a length of tape long enough to span the ends of cut.
You can cut the tape in half lengthwise if you are using 2" or more UV resistant tape. Pieces can overlap on the cut and do not have to meet on the hinge. You should be able to lift the sealed bottle by the neck without the cut opening up.

Step 7: Labelling

  1. Cut a 1 inch by 2 inch strip of the aluminum.
  2. Use the scissor's grip to smooth the strip of pattern or brand imprint.
  3. Punch a hole in one end with the skewer or an awl.
  4. Put the label strip on a pad of newspaper and print the name of the plant whose seed you're starting in the bottle.You can use the skewer or awl tip or an old stylus to do this. The important thing is to make an impression in the metal that will not be affected by the sun.
  5. Cut a 6 inch length of wire. Thread it through the tag's hole and twist the wire to form a small loop at the tag.
  6. Secure around the top of the bottle.

Step 8: You're Done!

Put this outside in early January. Last winter I had a dozen such bottles wedged in a wire basket with 6 inch sides. They got plenty of light and none of our blustery winter days carried them away! I began checking them for seedlings in April, but most varieties didn't sprout until May. Once they were up an inch or two, I began planting them out in my garden.

You can start them later in winter as long as there will be a series of freezing nights after you set them out.

I was able to reuse the bottles (and even the UV resistant tape) for this winter's CMS seeding after storing them in a closed container all summer and fall. The soil did need to be replenished and re-moistened.

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