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Hoop cover calculator for raised beds

A 4 × 8 ft bed with hoops 24 inches tall takes 3 hoops 4 ft apart, each 7 ft 3½ in of ½-inch conduit, one from each 10 ft length, and a cover 7.3 × 16 ft. Enter your beds to size every part.

Example: one 4 × 8 ft bed

3hoops of 7 ft 3½ in
3lengths of pipe
7.3 × 16 ftcover
24 in4 ft across
End on
3 hoops 4 ft apart along 8 ft
From the side

What to buy

  • 3 lengths of ½ in EMT conduit, 10 ft long. 3 hoops of 7 ft 3½ in: one from each 10 ft length, 2 ft 8½ in left over
  • 6 pipe straps or stakes, one at each foot: a strap screwed inside the bed wall, or a metal stake pushed in beside the hoop
  • 7.3 × 16 ft of row cover or clear plastic: the hoop's 6.3 ft curve plus 6 in at each side, the bed's 8 ft plus 4 ft at each end
  • 3 ties over the cover, one at each hoop, each about 9 ft of twine, rope or bungee cord

Clipping the cover on instead of tying it? Clips are sold by pipe size: ½ in EMT conduit is 0.706 in across the outside, ½ in PVC pipe 0.84 in.

Example: one 4 × 8 ft bed with hoops 24 inches tall, a half circle. Replace it with your beds.

Oregon State builds low tunnels 2 to 3 ft tall at the top of the hoop, for crops under 2 ft when grown: smaller hoops for short crops such as lettuce, taller ones for crops such as eggplant. Set the height to clear your crop at full size.

How long each hoop is

Each hoop is drawn as half an ellipse across the bed: a half circle when it is as tall as half the bed’s width, as 24 inches is on a 4 ft bed, which makes 75 inches of curve. The length that goes into the soil or a socket at each foot is added to that. A 3 ft bed with hoops 2 ft tall takes hoops of 6 ft 6½ in; a 4 ft bed, 7 ft 3½ in. Oregon State University Extension cuts one 10-ft length of conduit to 6 to 8 ft for a bed 3 to 4 ft wide, the same range.

Conduit holds its curve once bent, and a pipe bender gives every hoop the same shape. PVC pipe bends by hand, but the University of Illinois Extension warns that PVC reacts with greenhouse polyethylene film and degrades it quickly where they touch: use row cover fabric over PVC hoops, or conduit under plastic.

Spacing and anchoring

Oregon State places hoops every 4 to 5 ft along the bed, and every 4 ft where it snows so the tunnel does not collapse. The University of Illinois Extension puts them at most 6 ft apart, closer in wind or snow. The calculator puts the first and last hoop at the ends of the bed and spaces the rest evenly, never wider than the spacing you enter.

Push the ends of each hoop at least 6 inches into the soil, Oregon State says, with a metal stake flush against each foot; on a raised bed you can screw a pipe strap to the inside of the wall at each foot instead, as Illinois suggests. Tie the cover down at each hoop with twine, rope or bungee cord, and leave 4 to 8 ft of cover past each end of the bed to gather and tie to an anchor.

The cover

Spun row cover fabric and greenhouse polyethylene are the two usual covers. The width the calculator gives is the hoop’s curve plus what you leave at each side to tie or weigh down; Illinois finds ten-foot-wide rolls of polyethylene wide enough for most low tunnel hoops. Oregon State builds low tunnels 2 to 3 ft tall and keeps them for crops under 2 ft tall when grown, with smaller hoops for short crops such as lettuce and taller ones for eggplant, so set the height for what you will grow under it.

Row cover fabric is sold by its weight per square yard: the heavier it is, the more frost it keeps off and the less light it lets through. The University of Maryland Extension gives three grades:

Weight per square yardLight throughFrost protection
0.45 oz, light90 to 95%2°F
0.5 to 1.0 oz, medium70 to 85%4 to 6°F
1.5 to 2.2 oz, heavy30 to 50%Up to 8°F

The light grade is sold as an insect barrier and can stay on beets, snap beans and salad greens from seeding to harvest. In summer the air under a row cover can be 5 to 15°F warmer than outside, which Maryland warns leads to heat stress.

Next steps

Choose what goes under the cover with the planting layouts by bed size, build the bed itself from the raised garden bed plans, or count the watering parts with the drip line and parts calculator.

References