Window Box Calculator — Compost & Plant Quantities

The plant count here is per single row along the length — a 60 cm box fits about 5-6 bedding plants in one row. Boxes wider than ~18-20 cm can take a second, staggered row, so double the figure for the back row and offset the plants for a fuller display (trailing at the front, upright at the back).

Aim for at least 15 cm of internal soil depth for bedding plants, and 20 cm or more for herbs and larger plants. Deeper boxes hold more compost and dry out more slowly.

In summer, daily or even twice daily in hot weather. South-facing boxes dry out fastest. Mix water-retaining gel into the compost to extend the time between waterings.

Trailing petunias, lobelia, geraniums, fuchsia, herbs, and small bulbs. Choose plants suited to your aspect (sun or shade).

Use compost with water-retaining gel, add saucers if possible, and water morning and evening in hot weather. Self-watering window boxes are excellent for hot positions.

Window boxes and other containers are planted deliberately densely for instant, full impact rather than leaving room for long-term growth — the RHS recommends close planting for container displays. That is why the presets here (bedding 12 cm, trailing/herbs 15 cm, small bulbs 8 cm) are tighter than typical in-ground spacing.