plants growing indoors under grow lights

Seedlings growing in a container under grow lights. Photo: C. Carignan, UME

Updated: February 20, 2023

Containers for seed starting

Almost any clean container may be used for seed starting provided it allows for good drainage and is at least 2” deep. Save money by reusing cottage cheese and yogurt containers, milk cartons, aluminum pans, and clear clamshells from the produce department or deli.


cell packs for transplants
4 - cell pack and 6 - cell pack
black landscaping flat with drainage holes
Black flat with holes for drainage
green flat for seed starting
Green plastic tray with no holes
  • You can buy plastic trays (a.k.a flats) that are 10.5 in. X 21 in. X 2 in. deep and contain drainage holes. You can also purchase them with no holes. (These are great for minimizing the problem of water getting on the floor or the lights below. You just need to be careful not to over-water.) 
  • Numerous types of small pots and pellets (expand when you add water) made of compressed peat are on the market for starting seeds. 
  • Plastic cell packs (a.k.a. inserts or market packs) are square or rectangular plastic cells joined together and designed to fit into a plastic flat. The individual cells range in size from 1/2 in. to 4 in. in diameter.
  • Plug trays are sturdy one-piece plastic flats that are divided into individual cells.
seedlings planting in cups
Reused yogurt cups with drainage holes in the bottom

Growing media (medium) or "soil" for seed starting 

There are special types of growing medium for starting seeds. Growing media have three main functions: 1) supply roots with nutrients, air, and water, 2) allow for maximum root growth, and 3) physically support the plant.

soilless mix in a container
Soilless growing media

Purchase a soilless growing media (a.k.a growing mix, transplant mix, or potting mix). They are light and fluffy and formulated to produce uniform plant growth. They usually contain some proportion of sphagnum peat moss, perlite (small white "popcorn"), and vermiculite, and are generally free of diseases, weed seeds, and insects. These mixes are desirable because the peat moss holds water very well, yet the large pore spaces allow excess water to drain easily. Their high porosity also promotes quick and extensive root growth.

Use a standard, all-purpose transplanting mix. It is not necessary to purchase a special (finer) seedling mix. The latter is useful only for sowing extremely small seeds.

  • Avoid heavy, dense potting mixes that contain “forest products”.
  • Conventional mixes have lime and chemical fertilizer added; organic mixes use organic fertilizers and often contain compost. Some organic mixes are substituting coir and rice hulls for peat and perlite.
  • Don’t try to save money by using garden soil. It is too dense for the job and contains weed seeds and possibly pathogens.
  • If you grow a lot of seedlings and also do container gardening you might want to invest in a large bag or compressed bale of commercial soilless growing media. It is cheaper than buying the same amount in small quantities.

Peat moss and alternatives to peat moss

  • Peat is an organic substance formed from plants (principally sphagnum moss) that decompose very slowly in waterlogged soils (bogs).
  • Peat has been valuable in horticulture because its fibrous structure helps it retain a lot of water and air.
  • There is concern over the ecological effects of excavating peat moss.
  • Rising fuel prices have increased the cost and caused professional growers to look for alternative ingredients for growing media.
  • Ground-up coconut husk fibers (coir) are a popular alternative, though the sustainability of harvesting and shipping this material from the tropics is widely debated.
  • Some gardeners and small farmers have learned to blend finished, screened compost with commercial growing media.
  • Compost is heavier than soilless media (about 25 lbs. /cubic foot) but is less expensive (especially if homemade!) and supplies nutrients and other compounds that promote plant growth and health. Try mixing compost 1:2 with soilless media.

Read more on this topic: Peat-free potting mixes

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