Starting Seeds & Making Potting Mix

I got a sunburn a couple days ago and I’ve been telling everyone, because I am super proud of it. Not proud I got burned, of course (always wear sunscreen!), but proud that it was warm enough and sunny enough that I could be happily working outside without a sweater, sifting totally-thawed-out compost to incorporate into a potting mix (you may recall from our newsletter updates that our compost piles were frozen solid for a good portion of the winter).

We’re using this potting mix to fill the seed trays where we’re planting seeds for most of our spring and summer crops. We are planting from seed rather than plant starts because seeds are much more affordable, and there is a much wider variety of crops available in seed form. Why plant in seed trays rather than directly into the spot where the crops will grow to maturity? There are a couple reasons. It’s too cold to plant outside right now, so a short growing season like we have here necessitates taking advantage of indoor space to get a head start on our crops. Seed trays are ideal since they fit a large number of plant starts in a very condensed space that is easy to monitor. We get an average of 75 consecutive frost-free days here, which is about two and a half months, so utilizing indoor space in these early spring months is essential if we want to grow any tender crops that take longer than 75 days to reach maturity and continue producing. And though frost-tolerant crops are a wonderful way to extend the growing season, we like our tomatoes and melons here, too.

A good potting mix is important in getting seeds off to a good start. It should be well-aerated and light so that the seedlings aren’t smothered in heavy, wet soil, but should still be able to retain moisture. It also should have nutrients available for the seedlings, and the ability to retain those nutrients (note that potting mixes are not the same as soils and often don’t contain any soil at all). Water-holding capacity is especially of concern when planting in seed trays, since the cells tend to dry out rapidly. If the potting mix is too dry, it can become hydrophobic (this happens with soil, too), meaning that water just pools or runs off, but doesn’t absorb into the soil no matter how much you water. It’s easiest to just avoid this by never letting your soil or potting mix get that dry.

However, the topsoil we used in our potting mix hadn’t been used in years and was completely bone dry, so we added some water to the mix and squeezed the parts that actually absorbed moisture together with the dry parts. This forced moisture into the pores of the dry elements, and the mix will be able to absorb water much better in the future. If the potting mix can’t absorb water, the seeds won’t germinate, and if they do, they’ll wither fast. At the end of this post, I’ve provided the potting mix recipe we used this year, but you can easily buy potting mixes at garden supply stores, too.

Before actually planting the seeds, I washed all the trays in a large tub of water diluted with a couple capfuls of bleach to kill any fungi or viruses that may have been camping out in the trays since we last used them. I filled the trays with the moist, but not muddy, potting mix, tamped this down a bit, and created a hole or two in each cell of the depth recommended for each seed. Your little finger is a good way to measure quickly while making holes—the fingernail is between ¼ and ½ inch. Then just placed the seeds in the holes, gently covered them back up, and watered lightly but thoroughly. The seed trays are placed on the beds in our attached greenhouse and covered in low tunnels to keep heat and moisture close, but if you don’t have a greenhouse, anywhere in your home that stays relatively warm (not hot) is fine, as long as you move the trays to a window when the seeds germinate.

We labeled each tray with the respective seed name and included the date they were planted so that we can monitor how long it takes them to germinate and how long we grew them in trays before transplanting. We also keep this information in a spreadsheet, along with the seed source, the date on the seed packet, planting location, and amount harvested. Always keep good records so you know how to do better next year!

Figuring out when to start particular seeds is not always intuitive, so I’ve been working with my coworker Tammy Howard, a horticulture specialist for ATTRA, to develop a planting schedule and rotation plan for the SIFT farm. She has 10 years of farming experience under her belt, and, well, 10 years ago I was attempting to navigate seventh grade, so it’s been a really helpful learning experience for me. As I’ve come to understand it, the basic idea is to first start the plants that take the longest to mature, or that you want to harvest earlier in the season, and progress with the plants that take less time to reach maturity (days to maturity for each plant is listed on the back of the seed packet). For a continuous harvest of something like greens or carrots, schedule multiple plantings a few weeks apart.

Some crops, especially root crops, don’t do well when started in trays and then transplanted, so these need to be started later when it’s warm enough to plant seeds directly in the ground. And when you’re planning where to plant certain crops, besides considering light availability and temperature, take rotation into account. Rotating plant families between beds from year to year reduces pest buildup in the soil—for example, don’t plant broccoli and cabbage (both in the Brassica family) this year in the bed where they grew last year, since that will encourage Brassica-specific pests to get real cozy in that bed.

We’re expanding the number of outside plots this year to five (up from just one active plot last season), and though we’re starting a few of the new ones out with only cover crops to improve the soil there, we’re also planning on significantly expanding production and farmers market sales this year. As a result, I’ve been seeding furiously all month and plan to be doing so through April (and then transplanting through June). It’s a lot of work, but I love that I get to be out in the greenhouses again.

We’ve been seeding more than we could hope to plant, so we’re hosting a plant sale May 31 and June 1, where vegetable, herb, and flower starts will be available (a bit of a shameless plug, I admit). If you’re in the Butte area, need some local sources of plants for your summer garden, and want to support your friendly local small-scale farm, please stop by! More details can be found on our calendar page.

Our thyme and oregano starts gettin’ pretty for the plant sale in Camille’s homemade heated seedbed

As promised, our 2013 potting mix recipe, to be amended in the future as needed:
1 five-gallon bucket finished compost (for nutrients and water retention)
1 five-gallon bucket topsoil (for structure and volume)
½ bucket recycled glass sand (for drainage; regular builder’s sand works just fine, but this was what we had on hand)
½ bucket rice hulls (for air space and water retention; we are trying to reduce our use of mined materials so this is kind of an experiment– vermiculite and perlite are more conventionally used for this purpose)
1 cup worm castings (for nutrients and organic matter; worm compost is high in nitrogen)
½ cup garden lime (for reducing soil acidity)

See Potting Mixes for Organic Production for more information on potting mixes and how to make your own.

Posted on: March 28th, 2013


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