How to Fertilize Onions Under Plastic Mulch


Extend the growing season by warming the soil with plastic sheeting in the early spring. According to the Pacific Northwest Extension publication, a gardener can raise the temperature of the soil more than 15 degrees Fahrenheit within two to three days with the installation of two sheets of clear plastic over the soil. Because onions are a common early spring vegetable, many gardeners plant onions under plastic mulch. Help your onions grow large and robust with regular fertilization, even while the plastic sheeting is in place.

Step 1

Work the garden soil as early as possible in the spring with the garden spade. Use the spade to cultivate the soil approximately 5 inches deep. Sprinkle the granular fertilizer over the soil, consulting the package recommendations for the amount of fertilizer to use for the size of your planting area. Smooth the top of the soil with the rake.

Step 2

Lay the two sheets of clear plastic over the planting area. Secure the edges and corners of the sheets with the bricks or large rocks to hold the plastic down taut on the soil surface.

Step 3

Mark the rows you will make with the permanent marker, drawing lines for rows between 1 and 2 feet apart. Cut along the lines on the plastic sheeting with the utility knife or the scissors to enable you to plant the onions.

Step 4

Plant the onion seeds approximately 2 inches apart along the rows and cover the onion seeds with ½ inch of soil. Water the seeds generously immediately after planting them.

Step 5

Thin the seeds when the tops are approximately 2 to 3 inches high by pulling the weakest plants. Make the final onion spacing approximately 6 inches between each onion plant.

Step 6

Watch the onions as they begin to swell beneath the surface of the soil. When you notice bulbs growing, wait approximately two weeks, then apply liquid fertilizer. Mix the fertilizer with water according to package recommendations and pour the fertilizer carefully onto the soil immediately beside each onion plant.

Things You'll Need

  • Garden spade
  • Granular fertilizer (10-10-10)
  • Rake
  • Onion seeds
  • 2 sheets of clear plastic
  • Permanent marker
  • Scissors or utility knife
  • Bricks or large rocks
  • Balanced liquid fertilizer (water-soluble)


  • Pacific Northwest Extension: Short Season Vegetable Gardening
  • Organic Gardening: Liquid Fertilizer
Keywords: clear plastic mulch, plant onions plastic, plastic mulch, fertilize onions plastic

About this Author

Kathryn Hatter is a 42-year-old veteran homeschool educator and regular contributor to Natural News. She is an accomplished gardener, seamstress, quilter, painter, cook, decorator, digital graphics creator and she enjoys technical and computer gadgets. She began writing for Internet publications in 2007. She is interested in natural health and hopes to continue her formal education in the health field (nursing) when family commitments will allow.