Seed starting containers need to be sterile and have good drainage.
Their three main jobs are to:
Start seeds in individual containers rather than open flats. If you start tomato seeds in open flats, you’ll need to split plants after they germinate and separate their root systems, which stresses new tomato seedlings. Transplanting is much easier when each sprout is contained in its own cell or cup. Seed cell trays are ideal and can be re-used year to year.
Calculate volume. If you’re starting a lot of tomatoes (and other crops), use flats with individual cells to accommodate large numbers of plants.
Set your containers in a tray, plastic box, or rimmed cookie sheet for balance and good drainage. A tray also lets you water from the bottom. That helps your seedlings produce strong root systems. Roots will need to reach down into the growing medium for water.
Sterilize plastic containers that you re-use from last year in a 5-10% bleach solution. You’ll help keep nasty bacteria and fungi away.
Go simple and cheap. There’s no need to go fancy with starting containers. Use something clean with good drainage. Remember to poke holes.
Try Styrofoam or plastic cups – gardener’s favorites. Poke holes in the bottom for drainage.
Recycle yogurt containers, milk cartons, margarine tubs, plastic jugs, egg cartons, or soda bottles. Wash thoroughly and poke holes in the bottom.
Purchase seed-growing flats with individual cells, also known as seed cell trays. They're re-usable from year to year.
Buy individual peat pots or peat pellets. You can transplant them into the next size pot or directly right in the ground.
Make your own! Cut newspaper strips 12” by 6”. Wrap several strips lengthwise around a soda can (or similar-sized aluminum can) – 4” up sides, 2” across the bottom – and press tightly. Remove the can. Poof! – you’ve got a tomato pot. Or use a nifty pot maker to make strips of ordinary newspaper into biodegradable seed starting pots. Before you pack pots close together in a tray for support, make sure you poke holes in the bottom.
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