At the Royal Horticultural Society's garden at Wisley in Surrey, the trial glasshouses built in 2007 are floored in herringbone-laid red engineering brick set on a sand bed over a compacted hardcore base. The floors have a slight crown to centre drainage channels. They are, on the morning of a June visit, swept clean and slightly damp from the morning watering.
Three miles away, at a private glasshouse belonging to a retired plantsman named Geoffrey Holm, the floor is bare earth, dressed annually with leaf mould, walked smooth by twenty years of passage between the benches. It too is swept and slightly damp. It performs, by most measures, equally well.
The question of what to put on a greenhouse floor is older than most modern gardeners realise and it has, over the centuries, attracted more considered opinion than the question of what to put in the greenhouse.
There are four traditional choices: poured concrete, gravel or crushed stone, brick or tile, and bare earth. Each has been used continuously somewhere in Britain for at least the past two hundred years. None has decisively won.
Poured concrete is the modern default, particularly for commercial glasshouses where wash-down hygiene and equipment loading are priorities. It is durable, level, easy to clean, and unsympathetic to slugs and worms. It also has, in a small home glasshouse, several drawbacks.
The first drawback is humidity. Concrete does not absorb water. The morning watering runs off the bench, hits the floor, and either drains away or stands in puddles. The greenhouse air, particularly in spring, can become drier than the plants want.
The second drawback is thermal behaviour. Concrete has, by mass, a similar heat capacity to brick or stone, but its smooth surface releases heat quickly into a cold morning and reheats slowly. A concrete floor on a clear March night will measure colder than a brick floor in the same greenhouse.
The third drawback is the worm question. A concrete floor is a worm-free zone. This will sound trivial. It is not. The worms that pass through a soil-floored greenhouse process organic matter, aerate the substrate beneath the floor, and contribute to a small but real ecological function. A concrete floor severs this.
Gravel or crushed-stone floors are common in mid-twentieth-century amateur greenhouses and persist in many British allotment plots. The gravel layer, typically two to four inches deep over a permeable membrane, drains rapidly, absorbs heat moderately well, and is forgiving of spills.
Its main weakness is comfort. A gravel floor is harder to walk on for long sessions, and chairs, kneelers, and benches do not sit level. It also collects organic debris in the interstices and becomes, over years, a slowly composting layer of leaf matter that can harbour fungal pathogens.
Brick or tile floors are the heritage choice and remain, in the considered opinion of most experienced greenhouse keepers, the best for a working home greenhouse. Brick has high thermal mass, moderate permeability, a sympathetic working surface, and a long visual life.
The brick floor's chief argument is the slow release of heat. A brick floor that has absorbed heat all afternoon will continue to release it for hours into the evening. The temperature lag is meaningfully different from concrete. Measurements at Wisley show the brick floor running 4 to 6 degrees Fahrenheit warmer than the air at 8 p.m. on a spring evening.
Brick also accommodates damp without trouble. Water that lands on the brick wicks into the joints and evaporates slowly, contributing to the humidity that most greenhouse crops, particularly tomatoes and cucumbers, prefer.
The cost of a brick floor is, however, considerable. A 6-by-8 greenhouse floor in herringbone engineering brick will cost roughly £900 in materials and a week of competent bricklaying in 2026. Most amateur gardeners decline.
The cheaper compromise, used by Geoffrey Holm and by many traditionally minded plantsmen, is bare earth dressed annually. The earth is dug over once a year, in November, levelled with a rake, and topped with an inch of sieved leaf mould. The result is a soft, dark, level floor that absorbs water, holds heat, and supports earthworm activity.
Bare earth has the additional advantage of being free. It also has the disadvantage of getting on shoes, blowing onto bench surfaces, and providing a habitat for slugs, which Geoffrey controls with nightly patrols and a small pair of long-handled tongs.
Geoffrey's greenhouse, by his accounting, has hosted three generations of slugs and has never been seriously damaged by them. The slug, he argues, is more pest in theory than in practice in a closed greenhouse with daily attention.
The decision among the four floor types is, in the end, a matter of values. The gardener who values hygiene and easy washing will choose concrete. The gardener who values low cost and quick installation will choose gravel. The gardener who values long thermal performance and visual beauty will choose brick.
The gardener who values the small ecological functions of soil and the lowest possible installation cost will choose bare earth, accept the slug, and dress the floor with leaf mould in November.
Wintergreen visited eleven small greenhouses across Surrey and Hampshire in May. The floor breakdown was: four bare earth, three brick or tile, two gravel, two concrete. The bare-earth and brick gardens reported the highest satisfaction. The concrete gardens reported the highest convenience and the lowest pest pressure.
There is no correct answer. There is only the floor the gardener can live with for the working life of the structure, which, given the cost and disruption of replacement, will be most of the gardener's own working life.
Geoffrey Holm, having decided in favour of earth in 1986, has not revisited the question in forty years. He sees no reason to start now. The greenhouse, he says, is doing what it should. The floor, he says, is doing what it should. He brings in his cucumbers in July and lets the worms work the rest.