Row cover is the cheapest season extension there is, and it is the piece of equipment most likely to be underused by someone who already owns a greenhouse. The instinct is that a greenhouse makes covers redundant. In practice the opposite is true: covers used inside a greenhouse are what turn an unheated structure into one that produces through winter.
The mechanism is straightforward. A cover does not generate heat. It traps a thin layer of air around the crop, reduces convective loss to moving air, and, importantly, blocks radiant loss to the night sky. On a clear calm night a plant surface can fall several degrees below the air temperature purely by radiating to space, and a cover intercepts that.
What each weight actually buys
| Weight | Frost protection | Light transmission | Main use |
|---|---|---|---|
| Very light, insect mesh | Almost none | 85 to 90 percent | Pest exclusion in the growing season, not frost |
| Lightweight, about 0.5 oz | 2 to 4 degF | About 85 to 90 percent | Season start and end, and light enough to float directly on the crop |
| Medium, about 0.9 oz | 4 to 6 degF | About 70 to 80 percent | The general purpose weight, and the one to own if you own one |
| Heavy, about 1.5 oz | 6 to 8 degF | About 50 to 70 percent | Overwintering hardy crops, and short-term protection during a cold snap |
| Very heavy or doubled | 8 to 10 degF | Under 50 percent | Emergency protection only, because the light cost is unsustainable |
The trade in that table is the whole subject. Every additional degree of protection costs light, and in the season when you most want the protection the light is already scarce. A heavy cover left on through a dull spell can cost more in growth than the frost it prevented would have.
That is why a Agfabric Frost Blanket, 0.55 oz, 10 x 100 ft ($32.61) at the lighter end is usually more useful in practice than a heavier fabric: it can stay on for extended periods without starving the crop, and a second layer can be added for the specific nights that need it.
The double-cover method
This is the technique that makes an unheated greenhouse genuinely productive in winter, and it comes from commercial season-extension practice.
The greenhouse or tunnel is the first layer. Row cover laid over hoops directly above the crop, inside the structure, is the second. The two together commonly hold a crop through nights 8 to 10 degrees Fahrenheit colder than either would alone, because the inner cover is protected from wind and is working in air that is already warmer than outside.
The practical effect is roughly a zone and a half of climate. Hardy greens that would survive outdoors in a mild winter survive under double cover in a considerably harder one, and the whole approach is worked through in winter greens in an unheated greenhouse.
Floating versus hooped
A cover laid directly on the crop is called a floating cover, and lightweight fabrics are designed for it. The crop lifts the fabric as it grows. It is quick, it needs no structure, and it works well for low crops sown in rows.
It has two limitations. Anywhere the fabric touches a leaf, that leaf is in contact with a cold wet surface, which reduces the protection at exactly the contact point and can cause damage in a hard frost. And on a taller or more brittle crop the weight of wet fabric or snow crushes growth.
Hoops solve both. A set of Gardener's Supply Garden Hoops Grow Tunnel Support ($44.99) or a complete HEKIWAY Frost Cloth Kit with 7 Garden Hoops ($59.99) holds the fabric clear of the foliage, creates a genuine air space, and makes access far easier because the cover can be lifted from one side rather than dragged off. Over an existing bed, an OOTONAT 8 ft Fiberglass Garden Hoop Tunnel Kit ($17.09) does the same job at low tunnel scale.
Securing it, which is where most covers are lost
A cover that lifts in wind is a cover that is not protecting anything, and it usually takes the crop with it. Row cover is extremely light and presents a large area, so securing the edges continuously matters more than it appears to.
- Bury or weight the whole edge, not points along it. Wind finds any gap and inflates the cover from below.
- Use Sandbaggy 6 in Landscape Staples, 200 Count ($54.97) at close spacing through a folded hem into firm ground. A folded edge resists tearing far better than a single thickness.
- Leave slack. A cover pulled drum tight has nowhere to go when it fills with air, and it tears at the fixings. A little slack lets it move.
- Weight the ends. The ends of a hooped run are where wind gets in, and they need closing rather than just tucking.
- Take it off before a gale. A cover is not worth losing a crop or a tunnel for, and it goes back on in ten minutes.
Covers in the growing season
Two other uses are worth knowing, because they use the same materials.
Pest exclusion. Very fine mesh such as Ultra-Fine Garden Mesh Insect Netting, 10 x 50 ft ($27.99) over a crop physically prevents adult insects from reaching it to lay eggs, which is the only truly reliable control for several pests. The catch is that it also excludes pollinators, so it comes off any crop that needs them once flowering starts.
Shade and heat protection. A light cover over newly transplanted seedlings for a few days reduces transpiration stress and improves establishment considerably. It is doing the same job as the shading described in cooling a greenhouse in summer, at plant scale rather than house scale.
Row cover and support
Row cover moves the transplant date earlier and the last harvest later, which changes every date on a sowing schedule. Recalculate them with the frost date planting calculator, which applies the protection shift directly.