Why autumn is the best time to build raised garden beds

Raised beds are a popular choice for growing vegetables, but their success depends less on the structure itself and more on the growing medium placed inside them.

Best time to build

Experts say the ideal period for constructing and filling raised beds is autumn, roughly from September to November. Materials added during this time have several months to settle and begin the biological processes that make soil fertile. Beds built and filled in autumn provide plants set out the following spring with a noticeably better environment than beds filled and planted in the same week.

If autumn has already passed, beds can still be built in spring, but expectations should be realistic. Lettuce, radishes and herbs tend to grow well in beds filled and planted in April or May, while nutrient-hungry plants such as courgettes, pumpkins and cabbage-family crops benefit from an additional season of soil development or extra organic matter added at planting time.

Getting the soil mix right

Simply mixing topsoil with compost is often not enough. It is recommended to combine quality compost, good topsoil and biochar, which helps the soil retain moisture during dry periods while maintaining drainage during wetter months. Soil built this way tends to keep improving over time rather than degrading.

Size and base considerations

Bed width should generally not exceed 1.2 metres, allowing gardeners to reach the centre from either side without stepping in and compacting the soil. Paths of at least 60 cm are recommended between beds. Depth should be at least 25–30 cm for most vegetables, while root crops such as carrots, parsnips and beetroot need 40 cm or more. Plastic weed-suppressing membrane is not recommended at the base of a raised bed, as it hinders drainage and earthworm activity; cardboard or compostable material is a better alternative.