A cover buys cool-season crops a week or two at the ends of the season.
As recorded: A row cover, low tunnel or cold frame lets cool-season crops go in earlier in spring and keep growing later in autumn: about a week earlier under a row cover, and about two weeks at each end under a low tunnel or a cold frame.
A good hunch A person has checked it against its sources. If you skip this, it still grows, just less well. The planner suggests it.
How it works
Soil warmed by the sun by day is the heat source at night; a covering low to the ground traps that heat around the plants on calm, clear nights, keeping them a few degrees above the open air through the light frosts at each end of the season.
What to do about it
Sow or set out cool-season crops under the cover that many weeks ahead of the open-ground date, and keep autumn crops covered past the usual last call. Vent on sunny days, keep plastic off the leaves, and close the cover before sunset.
How much it matters: The sources give weeks as ranges (cold frame two to four at each end, a plastic-covered frame two to six or more, row cover a week or two in spring); the rule stores the low end of each. No yield figure is stated or estimated.
The evidence
Evidence: checked against 6 sources in the research-based literature.
The 6 sources
The ground and frame of the cold frame are heated by the sun during the day and radiate that heat during the night, keeping the inside warmer. They are used to extend the growing season by two to four weeks in both the fall and the spring, allowing you to start plants outdoors earlier and continue growing them later in the season.
In trials in Fort Collins, Colorado, a plastic cover on a frame typically provides 3°F to over 6°F of frost protection. It works well for cool season crops that are somewhat tolerant of frosty nights and adds two to six weeks or more on both ends of the growing season.
Soil warmed by the sun in the daytime, is the source of heat for frost protection at night. Moist, smooth soil absorbs more heat. To trap heat from the soil around young vegetables at night, place a covering that is low to the ground and spreading. To recharge the heat source for the next night, any covering must allow sunlight to shine through to the soil or must be removed in the daytime.
Low = Plant 1–2 weeks before suggested planting date; Medium = 2–4 weeks before suggested planting date; High = 4–8 weeks before average planting date
Depending on the weight of the covering you choose, you can gain between 2 and 8 degrees of frost protection or warm the local environment sufficiently to harvest certain crops a week or two early.
Clear plastic is useless in providing frost protection because the radiant energy released from the soil on those frosty nights passes right through the covering and never warms the air around the plants.
Where this rule stops
Cool-season crops only. Calm, clear (radiation) frosts only: in an advective freeze protection is questionable. Row cover moves spring only (no source read gives an autumn figure). Degrees of protection are not stored: the sources range from 2 to 10 °F for the same fabric and disagree on clear plastic at night.
Plants it applies to
- American persimmon
- Anise hyssop
- Apple
- Apricot
- Arugula
- Asparagus
- Autumn olive
- Beet
- Black chokeberry
- Blackberry
- Blueberry
- Cabbage
- Calendula
- Carrot
- Celery
- Cereal rye
- Chamomile
- Chives
- Cilantro
- Crimson clover
- Crocus
- Currant
- Daffodil
- Dill
- Elderberry
- Endive
- False indigo bush
- Fava bean
- Feijoa
- Fennel
- Fig
- French tarragon
- Garlic
- Grape
- Hairy vetch
- Hardy kiwi
- Honeyberry
- Horseradish
- Kiwifruit
- Lacy phacelia
- Lavender
- Leek
- Lemon balm
- Lentil
- Lettuce
- Lovage
- Mustard greens
- Oats
- Olive
- Onion
- Oregano
- Parsley
- Parsnip
- Pea
- Peach
- Pear
- Plum
- Pomegranate
- Potato
- Purple coneflower
… and 24 more.
How this rule has changed
Every change is dated, newest first. The full record, with each reason, is in the data download.
- A person read the sources and confirmed it
- Added to the rules
Cite this page
Milpa Gardens. "A cover buys cool-season crops a week or two at the ends of the season." Evidence grade: A good hunch. https://milpa.garden/rules/row-cover-low-tunnel-cold-frame-lets/
A garden planner built on graded evidence. Every claim states its evidence grade and links its sources on the page. Cite it with the grade, and add the date you read it.
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How we grade evidence
- Well established
- The rule names its mechanism and is settled, by sources a person has read and checked or by plain arithmetic and geometry.
- Promising
- The mechanism is named and the literature backs it. A few questions are still open.
- A good hunch
- Plausible by its mechanism, but field studies are thin, so treat it as a lean.
- Contested
- The evidence points both ways, or no study has tested it, so the planner does not enforce it.
- Refuted
- The claim did not survive its sources. It is kept so the folklore can be answered.