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How to plant a window box so water can leave

Updated: October 2026

The trough is on the table and the plants are still in their nursery pots. How to plant a window box, in the order that avoids a soggy failure, is to confirm the drainage, fill with peat-free compost, then decide if a saucer stays on. Compost on a sealed base is a bucket. This page is from the Products Compared editorial team. The order follows Royal Horticultural Society guidance. Product notes are amazon.co.uk listings saved on 15 October 2026. We did not run a compost trial. A qualifying purchase through our links can earn a commission at no extra cost to you. Listings are on the [window box comparison](/hub/door-and-window-security/window-box).

The trough is on the table and the plants are still in their nursery pots.

Our top picks

B2C B2C rustic wood-effect window box

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B2C B2C rustic wood-effect window box
5.8/10about £12.94
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Bamworld Bamworld 7-pack red window boxes

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Bamworld Bamworld 7-pack red window boxes
7.3/10about £16.14
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ARTKETTY ARTKETTY 30 cm ceramic trough

Rank 3

ARTKETTY ARTKETTY 30 cm ceramic trough
8.3/10about £18.99
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On this page
  1. 1.Make the water leave before you add compost
  2. 2.Peat-free potting compost, not garden soil
  3. 3.Set the plants, firm, and water
  4. 4.Saucers in summer and off in winter

Make the water leave before you add compost

Turn the trough over before you open the compost. The RHS treats drainage holes as essential. Leave a small hole clear. Cover a hole with a crock only when it is larger than about 2.5 cm across. Do not add a deep gravel layer. Whitefurze’s 2×60 cm trough ticks drainage and does not describe holes. The 90 cm listing does not state holes. B2C supplies stoppers and marked spots. If the base is sealed, stop. Punching the trough is not drilling a uPVC sill. See the window box buying guide. Call a professional if the safe fix means drilling masonry above head height.

Peat-free potting compost, not garden soil

Use a bag that says peat-free. The RHS container pages specify that, not soil from the garden. Border soil slumps in a shallow trough. Peat-free multi-purpose compost suits bedding and a short crop. A soil-based peat-free mix, in the style of peat-free John Innes No. 3, suits a plant that stays longer. No brand is named here. Fill to just under the rootballs and firm the gaps only. Which box to buy is on best window box.
B2C B2C rustic wood-effect window box

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B2C B2C rustic wood-effect window box
5.8/10about £12.94
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Set the plants, firm, and water

Leave the plants in their pots while you arrange them, taller at the back if the street will see the box. Tip each plant out, fill around it, and firm the gaps so the rootball does not rock. Water until moisture reaches the holes you confirmed. The RHS says to water thoroughly when the compost needs it, rather than little and often. Fit a saucer only if you will empty it. A saucer on a sealed base does not create drainage. If the trough is Fuyuleyo or Kigima, read it as a reservoir. Do not drill it to add an exit. That choice is on self-watering or drainage.

Saucers in summer and off in winter

In summer the RHS says a small container in full sun may need water up to twice a day. That is a ceiling for a hot spell, not a quota. Empty the saucer a few hours after watering. Before winter, take the saucer off and raise the trough on feet if you can. Waterlogging in cold weather rots roots. Do not water if frost is forecast. Evergreens may still need a drink in a dry spell. Call the building manager if the trough would stop an escape window.

Common questions

What if the base has only pin-pricks or marked spots?
Treat it as closed until water can leave through a hole you can see. Whitefurze’s 60 cm listing ticks drainage and does not describe holes, and UK reviews often found pin-pricks or corner screw holes. The 90 cm listing does not state holes. B2C gives stoppers and marked spots. Stop before compost goes in. Punching the trough is not a licence to drill the uPVC sill.
When do crocks block the hole?
The RHS says to cover a hole only when it is larger than about 2.5 cm across. A smaller hole should stay clear, or the crock becomes the blockage. A deep gravel layer works against that limit, so this method skips it. If the trough has no hole, a crock cannot invent one. Confirm the outlet, then add compost.
Can you use garden soil in a window box?
Use peat-free potting compost. Multi-purpose peat-free suits bedding and a short vegetable crop. A soil-based peat-free mix, in the style of peat-free John Innes No. 3, suits a longer-term plant. Soil dug from a border slumps and drains poorly in a trough. The RHS container pages specify potting compost. No brand is recommended here.
How full should the compost be?
Set each rootball about 5 cm below the rim. That is the RHS lip, so water has somewhere to sit and compost is less likely to wash out. A BBC page uses about 2 cm, which is a tighter lip, not a rival recipe. Use 5 cm unless it would bury the stems. Firm the gaps only.
How often do you water in summer?
The RHS ceiling is up to twice a day for a small container in full sun. It is not a daily quota. South- and west-facing sills dry faster. Water thoroughly when the compost needs it. If you will not empty the saucer a few hours later, do not fit it.
When does the saucer come off?
Take it off before winter so the box is not standing in water. Raise the trough on feet if you can. Evergreens may still need water in a dry, covered spell. Dormant plants seldom do. Do not water if frost is forecast. Fuyuleyo and Kigima are planted as reservoirs, not as open troughs, and neither is a promise you can leave the box through a hot holiday.

Related pages

Windowsill herbs in a window box

Window boxes 2026

Best window box: size and holes that match the sill

Window box buying guide: measure, holes and fixing

Self-watering window box vs drainage: which to buy

Sources

Public UK sources and the amazon.co.uk listings in this set. Last checked 15 October 2026.

  1. RHS: growing plants in containers (retrieved 15 October 2026)
  2. RHS: how to plant a container (retrieved 15 October 2026)
  3. RHS: peat-free choices (retrieved 15 October 2026)
  4. BBC Gardeners' World: self-watering planters (retrieved 15 October 2026)
  5. Consumer Rights Act 2015, section 9 (retrieved 15 October 2026)
  6. GOV.UK: General Product Safety Regulations 2005 (Great Britain) (retrieved 15 October 2026)
  7. Approved Document B, volume 1, dwellings (retrieved 15 October 2026)
  8. Garden News: how to plant a window box (retrieved 15 October 2026)