Cardboard Seed Trays for Starting Vegetables Indoors

There is something quietly satisfying about turning the week's recycling into next summer's harvest. A cardboard tube that held the loo roll, or a box that carried the groceries, can become a perfectly good seed tray — one you plant out with the seedling still tucked inside it. No plastic pots to wash, no roots to disturb, no cost. Here is how to make it work properly.

Why Cardboard Is Genuinely Good for Seedlings

Cardboard is not just a frugal substitute for proper pots. It has real advantages in the seed-raising season.

• Roots are not disturbed. Seedlings that resent transplanting — peas, beans, sweet peas, leeks, brassicas — can go into the ground in their tube, so the root ball stays intact.
• It is breathable and porous. Air reaches the compost, and roots that reach the sides are discouraged rather than circling endlessly.
• It breaks down in the soil. Once buried, the cardboard rots away and feeds the worm population instead of sitting in a drawer for a decade.
• It is free and always available. Most households generate a steady supply without trying.

One rule matters above all: use plain, uncoated cardboard. Avoid anything glossy, laminated, waxed or heavily printed, and always strip off plastic tape, staples, address labels and window film. If it feels shiny or you can peel a plastic layer off it, it belongs in the recycling bin, not the compost.

Toilet Roll and Kitchen Roll Tubes

These are the workhorses of cardboard seed raising. A toilet roll tube is roughly 4cm across and 10cm deep, which is an excellent root run for most seedlings. A kitchen roll tube cut in half gives you two more, and a longer one still.

Stand the tubes upright and pack them tightly together — touching, so they hold each other up — inside a waterproof tray. A plastic mushroom tray, an old washing-up bowl or a cut-down bottle base all work. Packing them snugly matters: loose tubes wobble and collapse when watered.

Fill each tube with moist compost, firm it gently with your fingers, and sow as normal. Then write the variety and the date straight onto the outside of the tube with a permanent marker. It saves a forest of little plastic labels and it cannot blow away.

Good candidates for tubes: peas, mangetout, broad beans, runner beans, sweet peas, leeks, cabbage, kale, broccoli, beetroot and chard. Anything with a taproot or a deep habit will thank you.

Boxes, Egg Cartons and Trays Made to Measure

Delivery boxes, shoe boxes and the sturdy trays that hold tinned goods are all useful, though you will need to cut them down to size.

• Shallow trays. Cut a box down to about 8–10cm deep. Line it with a plastic bag, prick a few drainage holes in the base, and fill with compost. This makes an excellent tray for pricking out seedlings or for a cut-and-come-again salad crop.
• Individual pots. Roll a single sheet of thin card around a jam jar, slide it off, then fold and tuck the base to close it. Slightly fiddly, but very good for courgettes and squashes, which hate root disturbance.
• Egg cartons. The bottom half is ideal for chitting potatoes in a light, cool room, or for microgreens and tiny lettuce seedlings. The lid makes a decent saucer underneath.

Whichever you choose, the principle is the same: drainage, a snug fit, and enough depth for the roots you expect.

Filling, Sowing and Watering

Use a peat-free multipurpose compost, moistened the day before so it feels like a wrung-out flannel. Fill each tube or tray to about a centimetre below the rim, firm lightly, and sow at the depth given on the packet — usually about twice the diameter of the seed. Cover with compost or a thin layer of vermiculite.

Then water from below. Stand the whole tray in a couple of centimetres of water for ten minutes and let the cardboard draw it up. Watering from above disturbs seeds, splashes compost about and can rot the tubes. Never leave them sitting in deep water for hours, or the cardboard turns to pulp.

Cover the tray with a clear lid, a sheet of cling film or a sheet of glass to hold in humidity, and keep it somewhere warm — around 18–21°C suits most vegetables. Remove the cover the moment the first green shoots appear, then move the tray to a bright windowsill. Turn it daily so the seedlings grow straight rather than leaning towards the light.

Planting Out

About a week to ten days before planting, start hardening the seedlings off: put them outside in a sheltered spot for a few hours each day, building up the time and bringing them back in at night.

Water the tray an hour before planting, then set each seedling out tube and all, buried right up to the rim. This last point is important. If the cardboard collar sits above soil level, it acts like a wick and dries the root ball out; buried properly, it stays damp and rots away happily. If roots have not yet reached the base of the tube, tear the bottom off gently first. Firm the soil around each plant and water well.

Common Problems and Quick Fixes

• Furry mould on the surface. Usually too wet and too little air. Scrape it off, water less, and improve ventilation.
• Floppy, collapsing tubes. They have been standing in water. Let the tray drain and water only from below, briefly.
• Tubes splitting as you fill them. The cardboard is damp before it is used, or you are firming the compost too hard. Keep spare cardboard dry until needed.
• Leggy, pale seedlings. Not enough light. Move them closer to the window, or into a cooler, brighter spot.
• Slugs and snails. They will happily nibble young cardboard. Set plants out when they are sturdy and patrol in the evenings.

Start with a handful of tubes on a windowsill this week and you will have a stock of sturdy, well-rooted plants ready for the garden — and a recycling bin that is noticeably emptier than usual.