Make greenhouse beds no wider than you can reach to the middle, about 120 cm wide when you can work from both sides, so you never step on the soil. Raised beds drain and warm sooner in spring but dry out faster, while beds in the ground hold moisture and need no walls.
A raised bed is soil held above ground level by a frame; a ground bed is dug into the existing soil with permanent paths beside it.
Bed Width Is Set By Your Arm, Not By The Greenhouse
The cold frame rule applies to every bed: don’t build wider than you can reach to the back row, or you will step on plants and compact the soil. A bed worked from both sides can be twice your comfortable reach. With a 60 cm reach from each side that is 2 × 60 = 120 cm. A bed worked from one side only is limited to a single reach, so a 120 cm cold frame is a stretch for most people; if you cannot reach the back row, make the box shallower. Measure your own reach and use it.
What You Need
- Soil and compost to fill or amend the bed — see Soil Management
- For a raised bed: edging of boards, logs, stone or earthbags (the same materials as Basic Box Construction)
- A mulch — see Mulching
- A fork or spade, and a line to mark straight edges
Use rot-resistant wood for edging, and avoid salvaged lumber that was pressure-treated or soaked in preservative where soil will grow food.
Method: Lay Out The Beds
Step 1: Mark the beds and paths. Make beds 120 cm wide with paths between them wide enough to walk and carry a bucket.
Step 2: For ground beds, loosen the soil. Fork the soil without turning it over more than necessary, and work in compost. Keep the paths unplanted so the beds stay uncompacted.
Step 3: For raised beds, build the frame and fill. Fill with a mixture of topsoil and compost. The raised soil drains freely and warms sooner in spring.
Step 4: Mulch the surface. A layer of mulch limits water loss and weeds.
Step 5: Water by the bed’s behavior. Raised beds dry faster and need watering more often; ground beds hold water longer. Check by pushing a finger into the soil.
Worked Example
A greenhouse 3.0 m wide and 6 m long. One central bed 1.2 m wide, worked from both sides (60 cm from each), leaves 3.0 − 1.2 = 1.8 m for the two side paths, so each path is 1.8 / 2 = 0.9 m wide. Bed area = 1.2 m × 6 m = 7.2 m². Check: 0.9 + 1.2 + 0.9 = 3.0 m. If you would rather fill the width with bed, use two beds along the walls, each no wider than your single reach, with a path down the middle.
Common Mistakes
- A bed too wide to reach — you step on it and the soil compacts.
- Treated or preservative-soaked wood as edging — the preservative leaches into food soil.
- Letting raised beds dry out — they drain freely and need more watering.
What’s Next
- Learn next: Succession Planting Schedules — what to put in the beds and when
- Combine with: Drainage — where the water in a heavy soil should go