The choice between beds and benches decides what a greenhouse is for. Benches hold trays and pots at working height and suit propagation, ornamentals and anything moved in and out. Beds hold soil in the ground and suit crops that stay put and grow large: tomatoes, cucumbers, peppers, and a winter greens rotation.
The practical threshold is width. Below about 7 feet, a house barely fits two benches and a path, and a bed at floor level makes the space feel smaller and harder to work. From roughly 8 feet up, a bed along one side with a bench opposite is a genuinely better arrangement, and above 10 feet two beds and a central path is the classic layout.
Depth, which is the decision that matters
Depth determines root volume and water reserve, and both of those determine how forgiving the growing is. A shallow bed under cover behaves like a large container: it dries quickly, it dries unevenly, and the watering schedule has to be far more frequent to compensate.
| Crop group | Minimum depth | Preferred depth | Why |
|---|---|---|---|
| Salad greens and herbs | 8 in | 10 in | Shallow fibrous roots, but the water reserve is what actually sets the depth |
| Brassicas and chard | 10 in | 12 in | Larger plants with a deeper anchoring root |
| Tomatoes, peppers, aubergine | 12 in | 16 to 18 in | Deep rooting, and blossom end rot is far less common in a deep bed because the reserve buffers water swings |
| Cucumbers and melons | 12 in | 16 in | High water demand at full canopy, which a shallow bed cannot buffer |
| Carrots and parsnips | 14 in | 18 in | The root is the crop, and it forks in anything shallower or compacted |
| Strawberries | 10 in | 12 in | Moderate rooting, but crown rot risk makes drainage more important than depth |
A bed sitting on open ground is meaningfully better than the same bed on a slab, because roots continue downward and drainage continues downward. That is one of the strongest arguments against a full concrete floor, covered in greenhouse flooring and paths.
Work out the fill volume before buying anything with the bed and soil volume calculator. The quantities surprise people: a single 4 by 8 foot bed at 12 inches deep takes 32 cubic feet, which is roughly 16 bags of 2 cubic feet, and at bagged-mix prices that exceeds the cost of the bed frame several times over.
Width, length and the path
Bed width is set by reach, not by the space available. Four feet is the practical maximum for a bed accessible from both sides, and two feet is the maximum for one accessible from one side only, such as a bed against a wall. Anything wider means standing on the bed, which compacts it and undoes the reason for building it.
Path width is set by what you carry. Twenty-four inches is the working minimum for walking through with a tray in both hands and is tight for a wheelbarrow. Thirty inches is comfortable. The trade is direct: every inch of path is an inch not growing, and on a small house the proportion lost is large. The greenhouse size calculator does that arithmetic explicitly.
Height above the floor is a separate question from depth of medium. A bed at ground level with 12 inches of medium and a bed raised 12 inches on legs with the same medium grow identically, but the raised one is far easier on the back and drains faster. A Cedar Raised Garden Bed with Legs, 48 x 24 x 30 in ($84.99) at 30 inches is a bed you can work standing up, which for anyone who finds kneeling difficult changes what is possible. A ground-level Land Guard 8 x 4 x 2 ft Galvanized Raised Bed Kit ($59.99) gives more depth for the same medium volume and keeps the connection to the soil below.
What to fill a bed with
A greenhouse bed is not a container and it is not open garden soil. It sits between the two, and the mix should reflect that.
A workable general fill is roughly half good topsoil or screened loam, a third compost or well-rotted organic matter, and the remainder something that keeps structure open, such as coarse grit, perlite or composted bark. Pure potting mix in a large bed is expensive, holds too much water at depth and slumps badly over a season. Pure garden soil compacts in the absence of freeze-thaw and worm traffic, both of which are reduced under cover.
For the top few inches where seed goes, a finer sterile medium such as Burpee Organic Coconut Coir Seed Starting Mix, 16 qt Expanded ($10.39) raked into the surface improves germination considerably over sowing into a coarse bed fill. The medium question in full, including what to use for trays and containers, is in greenhouse soil and growing media.
Expect settlement. A newly filled bed drops noticeably in the first season as organic matter breaks down and the profile consolidates, so buy roughly ten percent more than the calculated volume and plan to top up annually with compost.
The two problems a permanent indoor bed develops
Salt accumulation. Nothing under cover is ever leached by rain. Dissolved minerals from tap water and residues from fertiliser stay in the profile and concentrate year after year, until growth slows, leaf margins scorch and a white crust appears on the surface. The fixes are rainwater where possible, deliberate leaching once or twice a year with a large volume of water, and feeding less than a label written for outdoor use suggests. This is covered fully in watering in a greenhouse.
Soilborne disease build-up. A greenhouse bed grows the same crop in the same soil under near-ideal conditions for pathogens, with no winter to interrupt and no rotation to break the cycle. Tomato growers hit this first, usually in year three or four, as vigour drops for no obvious reason.
- Rotate within the bed as far as a small house allows. Even alternating between a fruiting crop and a leafy one from year to year interrupts several pathogens.
- Grow the susceptible crop in containers instead. Gardzen 5 Gallon Fabric Grow Bags, 10-Pack ($21.99) with fresh medium each season sidesteps the problem entirely, at the cost of more frequent watering.
- Use grafted plants. Grafting a susceptible variety onto vigorous resistant rootstock is the commercial answer to a permanent bed, and it is achievable at home with Parafilm M Grafting Tape, 1 in x 90 ft ($9.85) and a humid chamber for the week after grafting.
- Replace the top layer. Removing and replacing the top six inches every few years is laborious but effective, and it is far less work than emptying the bed.
- Clear debris ruthlessly. Old roots, fallen leaves and spent plants left in a bed are where next season problem overwinters.
Beds, containers and fill
Beds change the watering pattern, the thermal behaviour and the layout of a greenhouse all at once. Size the fill with the soil volume calculator, then check the demand it creates against the water use calculator before committing to a hand-watering routine you will not keep up.