How to grow herbs in a window box

Inspired by Huw Richards for Dobbies. This guide is based on “How To: Grow your own window box herbs”, a practical video walkthrough by UK gardening presenter Huw Richards for Dobbies Garden Centres. Huw covers herb selection, planting up a container, and keeping it looking good through the season — useful whether your window box is at ground level, on a sill, or fixed to a balcony rail.

1. Choose a container with drainage holes

A window box without drainage is a recipe for disaster. Check the base before you buy — if there are no holes, drill several yourself with a 10 mm bit. Terracotta breathes and dries out more evenly than plastic, which makes it a better choice for herbs. That said, plastic is lighter and fine for window-sill mounting where weight matters.

Size matters more than people expect. A box at least 20 cm deep gives roots room to develop. A longer box also lets you grow a better variety without individual pots getting crowded.

2. Pick herbs that share the same conditions

Not all herbs want the same things. Mediterranean herbs — rosemary, thyme, sage, and oregano — need full sun and sharp drainage and genuinely hate wet roots. They work brilliantly together. Mint, parsley, and chives are happy in partial shade and prefer more moisture. Mix these with Mediterranean herbs and one lot or the other will struggle.

Plant similar herbs together in separate boxes rather than mixing types. Two boxes — one sunny and dry, one shadier and damper — gives you more than one sprawling compromise that suits nobody.

3. Use the right compost mix

For Mediterranean herbs, mix general purpose compost with roughly one third horticultural grit by volume. This improves drainage dramatically and replicates the lean, stony soils these herbs come from. They grow less lushly but far more aromatically with it.

For moisture-loving herbs like mint and parsley, standard multipurpose compost on its own is fine. Add a little slow-release granular fertiliser to the mix at planting time, and you should not need to feed again until mid-season.

4. Plant generously but not too tightly

Start with young plants rather than seeds — a herb box planted with young plants gives results in weeks rather than months, which is the whole point. Space them far enough apart that air can circulate, but close enough to look lush from the start.

Pinch the tips of each plant as you go in. This encourages bushy growth from the base rather than a single leggy stem that flops over by the end of summer. It sounds counterintuitive to remove growth the moment you plant it, but it genuinely makes a difference.

5. Position the box thoughtfully

Fix the box securely before you fill it — a window box full of compost and plants is much heavier than it looks. Use proper window box brackets rated for the load, fixed into solid masonry or timber with the right fixings for your wall type.

If the box is on a south or west-facing sill, check it twice a day in summer heat. Compost dries out astonishingly fast in full sun. A saucer underneath helps retain a little moisture, but remove standing water after an hour or two.

6. Water carefully and consistently

The most common cause of herb failure in window boxes is overwatering. Push a finger an inch into the compost — if it feels dry, water. If it still feels slightly damp, leave it another day. Mediterranean herbs in particular recover from drought far better than they recover from waterlogging.

Water at the base of the plants rather than over the top in warm weather. Wet foliage in heat encourages fungal problems, particularly on basil, which is both the most popular and the most temperamental herb to grow in a container.

7. Harvest regularly to keep it productive

Regular harvesting keeps herbs bushy and productive. Never take more than a third of any plant at one time — it needs leaves to photosynthesise and bounce back. Snip herbs from the top and outside of the plant rather than pulling whole stems from the centre.

Remove any flowers as soon as they appear on basil, coriander, and chives. Once these herbs flower, they redirect energy into seed production and the leaves become smaller and less flavourful. Pinch the flowers off and the leaves will continue growing for weeks longer.

8. Refresh the box each season

Perennial herbs like rosemary, thyme, and mint will keep going year after year if you cut them back in spring and top-dress with fresh compost. Annuals like basil and coriander need replacing each season. Pull them out at the end of summer, refresh the compost, and start fresh the following year.

Mind you — a slightly neglected herb box is still better than none at all. Even one that has seen better days will usually have enough left in it for a few handfuls of thyme or a bunch of chives. Do not give up on it too soon.

When to call a handyman

If you want window boxes fixed to the outside of the house — especially at height, on a rendered or pebbledash wall, or where the fixings are going into old mortar — it is worth getting the brackets put up properly. A falling window box from a first-floor sill is not a small thing. Richard can fit the brackets securely and level.