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How to Make Your Own Compostable Seed Starters

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I tend to let my garden rest for the winter under cover crops, so when the time approaches to begin actively gardening in the spring, I get really excited. When there is still danger of frost, it can be hard to wait to get crops in the ground. What follows is a tutorial for basic compostable DIY seed starters to keep your hands busy and give your plants a jump start while you wait for last frost.

Materials for Compostable DIY Seed Starters

There are so many household items that can be repurposed as compostable DIY seed starters. Egg cartons are commonly touted as organic and easy to place in the ground compostable modules to get your plants started. However, I’ve found that they aren’t quite deep enough for some of the crops I like to start early.

Compostable DIY Seed Starter

For a deeper seed starter that is just as organic, compostable, and inexpensive you’ll need the following items:

  • Shallow cardboard shipping box
  • Ruler
  • Scissors or craft knife (If you don’t have a craft knife, I highly recommend this tool for all your craft and DIY purposes! Find an inexpensive craft knife on Amazon)
Making DIY compostable seed starting trays starts with a box.

Step 1: Choose a Cardboard Box for Your DIY Seed Starters

For a good compostable DIY seed starter, choose a box that is at least 2 inches deep. I like to diversify my options and create seeds starters from several different boxes depending on what I will plant in them. For tomatoes, which I let grow to about 6-8″ tall before planting, I go with a larger options for seed starters, around 4-5″ deep. For greens, I opt for the shallower option, and only 1-2″ of soil is necessary in my seed starters.

Step 2: Measure and Cut the Cardboard Box

Remove any labels and plastic tape from your box. Many companies nowadays use compostable tape for their boxes, which is fibrous and matte texture, not glossy. This kind of tape is fine to leave on your DIY seed starter.

The best feature of a cardboard box is that its flaps are pre-cut to the length and width of the box. Remove the top flaps from the box; if your box has compostable tape securing the bottom, you can also remove the inner bottom flaps (these are typically the shorter flaps).

Cut each of the flaps in half length-wise, using a ruler as a guide to measure.

Measure the length of each strip and cut slits halfway into the width of the cardboard. The spacing between slits will determine the size of each cell in your seed starter. The cuts should match between each size of cardboard flaps/strips you are cutting (e.g. all the shorter strips should be identical, and all the longer strips should be identical). Each short piece should have as many cuts/slits as their are long pieces and vice versa.

Assemble the strips of cardboard into a grid, securing the long and short pieces with the slits cut into each.

As an optional step, use a craft knife to score the bottom of the box along the lines of the grid you just created, but don’t cut all the way through. This will help you punch out the seedling plugs once you are ready to plant them in the ground.

Step 3: Fill Your DIY Seed Starters with Seed Starting Soil

A variety of seed starting soil mixes are available for purchase, but not all may be ideal for starting seeds. Potting soil is different from seed starting soil, and may be too coarse for your seedlings to flourish. Check out this article about seed starting mixes, and consider a recipe for creating your own seed starting mix.

The soil depth and planting depth of your seeds depends on what kind of crops you are growing. Some seeds might rest on the surface of the soil, but don’t need to be covered with soil at all. Check out this super helpful guide to seed planting depth.

Step 4: Germinate Your DIY Seed Starters in a Warm, Sunny Location

Place your compostable DIY seed starters in a warm location, perhaps by a window, in an area of the house that gets adequate sunlight. I place my seed starters under a grow light to give them the strength of light they need for a good start. Aim to

Keep your compostable DIY seed starters moist but not soggy. I place mine on a multi-purpose plastic tray to keep them from dripping. The cardboard will provide both drainage and moisture retention.

Step 5: Remove the Seedlings from Your DIY Seed Starter

If you scored the bottom of your seed starter box beforehand, this can be as easy as gently pushing each plug of soil and roots from the bottom. Plant the entire plug in your garden or planters.

Add the remaining cardboard to your compost pile, or place it between the rows of your crops as a compostable weed barrier.

Other Ideas for DIY Seed Starters

So many other household items can be repurposed as compostable DIY seed starters:

  • Paper towel or toilet paper rolls
  • Paper cups
  • Brown paper bags
  • Muffin tin liners
  • Egg cartons (and egg shells!)
  • Newspaper
  • Use plastic wrap and sticks to create a mini greenhouse
  • Use an aluminum cake tin with clear plastic cover to hold seed starters and create a greenhouse effect

For more gardening inspiration, check out our post on Whimsical Garden Ideas!

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