A glass house, and the wind that empties it
A greenhouse catches sunshine easily. Keeping that warmth in on a cold day is harder, and getting rid of it on a hot day is a job of its own.
A greenhouse is a trap for sunshine. The University of Alaska Fairbanks Cooperative Extension Service — which writes for gardeners in one of the coldest states in the country — explains the trap in one sentence: “As solar heat in the form of light enters the greenhouse, objects absorb it and reradiate it into the greenhouse environment (the greenhouse effect).” Light comes in through the clear roof; the soil, the pots, the benches and the plants soak it up and warm up. Keeping that warmth is the hard part. The same publication lists the ways it leaves — “Heat loss from a greenhouse occurs through convection, conduction or radiation” — and defines each one. Convection “is the transfer of heat through circulation and mixing of warm air with colder air.” Conduction “is the movement of heat between or through objects such as a metal rod or a greenhouse covering material.” Radiation “is the movement of heat from a warm to a colder object without the need for a medium such as air (convection) or direct contact between or through objects (conduction).” Then comes the surprise: “Although radiation is important for solar heating of a greenhouse, thermal radiation is of minor importance for heat loss from a greenhouse.” Moving air matters a great deal. “Wind, on the other hand, significantly increases heat loss,” the publication says, and even the gaps count: “Air exchange or infiltration through greenhouse cracks, small openings and gaps around doors, equipment and other greenhouse features, is another form of convective heat transfer.”
Basis: University of Alaska Fairbanks Cooperative Extension Service, publication HGA-00336, “Controlling the Greenhouse Environment,” section Heating. Quotations unchanged.
In summer the trap works too well. The same guide warns: “As the sun warms the greenhouse, temperature rises rapidly. Without a method of removing excess heat, the temperature may increase to levels detrimental for crop growth.” The simplest fix needs no machine at all. Passive ventilation systems, the guide explains, “operate on the principle that air expands when heated, increasing the pressure in the greenhouse,” and with such a system “hot air flows out of vents positioned in the greenhouse roof or end walls. Cooler air is brought in through side vents (or other uncontrolled openings) to replace the hot air.” Out at the top, in at the sides. Growers also shade, with “shading compounds (or diluted latex paint) applied to the outside greenhouse covering material or fabrics draped over or used as a curtain inside the greenhouse.” And moving air is not only about temperature: the guide gives a second reason airflow is needed — “to control relative humidity and carbon dioxide within the plant canopy.” Even the water drops on the inside of a plastic roof are a trade-off. Condensation, the guide says, “in addition to reducing thermal loss, also limits transmission of light and solar radiation.” Warmer, but darker.
Basis: University of Alaska Fairbanks Cooperative Extension Service, publication HGA-00336, “Controlling the Greenhouse Environment,” sections Heating, Shading, and Cooling. Quotations unchanged.
USDA has its own word for what a greenhouse does: cover. When the Census of Agriculture sorts farms into types, a greenhouse, nursery and floriculture farm is one “primarily engaged in growing crops of any kind under cover and/or growing nursery stock and flowers,” and the appendix spells out the word: “‘Under cover’ is generally defined as greenhouses, cold frames, cloth houses, and lath houses.” Crops of any kind. That is why the census section that counts flowers and shrubs also collects sales of “food crops grown under protection, and mushroom crops.” A greenhouse does not care whether it is warming a poinsettia or a tomato.
Basis: USDA NASS, 2022 Census of Agriculture, Appendix B, entries “Greenhouse, nursery, and floriculture production (1114)” and “Nursery, greenhouse, floriculture, and sod.” Quotations unchanged, except that the appendix’s doubled quotation marks around Under cover are shown as single quotes.
Why are the three states where these crops make up the biggest share of farm sales all in the Northeast? None of the sources used here says, and a share is a slippery thing to explain, because it has two halves. One half is the part: maybe plants for yards, parks and gardens sell well where many people live close to the farms that grow them, and a potted shrub or a roll of lawn is heavy and fragile enough that being close could matter. The other half is the whole: a state without much room for big grain fields or large herds has a smaller total to divide by, and any crop looks large inside a small total. Local history can push too — a 2021 ecoRI News report, citing University of Rhode Island professor emeritus Michael Sullivan, describes Rhode Island sod farms that had grown potatoes in the 1950s. Each of these is a possibility, not a finding. A column of percentages cannot tell you which half moved it.
※ A hypothesis is an idea that isn't proven yet.
📊 Sources for the rankings mentioned in this note (links to the original data and retrieval dates) are on each quiz page below.