Protecting plants from a late frost

What “late” is being compared with

A late frost is late relative to a date you can point at. On this site that date is a probability from a named station, not a hunch. At Austin-Bergstrom, USW00013904, the median last 32°F date is March 12, and the 10% last 32°F date is April 2. A freeze on March 20 is after the median and still inside the 10-to-90 band. A freeze on April 10 is past the 10% date, which is the uncommon side of the 1991–2020 distribution: the methodology says the 10% last-spring date is the date with a 10% chance the last freeze occurs on or after it. Neither date is a forecast for this spring. The National Weather Service forecast is the document that tells you a particular night is coming. The normal tells you whether that night is ordinary.

Protection is justified by the forecast and by the plant, not by the fact that the calendar has passed the median. Iowa State University Extension’s page “How to Protect Plants from Frost and Freeze” treats the forecast low as the trigger. It says that once temperatures dip to the mid-30s °F it is time to bring plants indoors, cover them, or use a cold frame or hoop tunnel. That sentence is about action on a named night. It is not a standing order to keep every bed covered for two weeks after the average last frost.

Which crops AgriLife times after the frost-free date

Texas A&M AgriLife’s home vegetable guide does not publish a column labeled “frost susceptible.” What it publishes, in Table 8 of the Texas Home Vegetable Gardening Guide, is a spring planting time relative to the frost-free date. The crops placed entirely after that date are the ones the guide does not want in the ground while freezes are still the normal expectation. In the table retrieved for this site they are:

• Snap beans, bush and pole, and lima beans, bush and pole: 1 to 4 weeks after.
• Sweet corn: 1 to 6 weeks after.
• Cucumber: 1 to 6 weeks after.
• Muskmelon: 1 to 6 weeks after.
• Eggplant: 2 to 6 weeks after.
• Okra: 2 to 6 weeks after.
• Southern peas: 2 to 10 weeks after.
• Peppers: 1 to 8 weeks after.
• Sweet potatoes: 2 to 8 weeks after.
• Pumpkin: 1 to 4 weeks after.
• Summer squash: 1 to 4 weeks after.
• Winter squash: 1 to 4 weeks after.
• Tomato: 1 to 8 weeks after.
• Watermelon: 1 to 6 weeks after.
• Mustard: 1 to 6 weeks after.

Mustard is on that list because the cell says so, not because this site reclassified it. Crops the same table places before the frost-free date include beets, broccoli, Brussels sprouts, cabbage, carrots, collards and kale, English peas, Irish potatoes, spinach, Swiss chard, and turnips. Lettuce is written across the date: “6 weeks before or 2 weeks after.” Radishes run from 6 weeks before to 4 weeks after. A cover decision follows the plant you actually have, not a slogan that all greens are safe and all fruiting crops are not. English peas are timed before the frost-free date. A tomato is timed after it. Iowa State’s pea page says the plants tolerate cool temperatures and can survive a light frost, and that flowers and developing pods are sensitive to freezes or frosts. A pea in flower is not the same risk as a pea seedling. The table and the pea note answer different questions.

AgriLife’s High Plains vegetable guide, a separate publication, adds a soil-temperature caution that is about emergence and transplant shock rather than about a cloth on a Tuesday night. It says most cool-season vegetables such as beets, broccoli, cabbage, lettuce, radishes, spinach, and leafy greens need a minimum soil temperature of 50°F, and that warm-season vegetables such as tomatoes, peppers, eggplant, cucumbers, squash, and sweet potatoes generally need soils around 65°F, checked at about 6 inches for about a week before seeding or transplanting. Those numbers are for the High Plains section of that guide. They are not a statewide substitute for Table 8, and they are not a reason to skip a cover if a freeze is forecast after you have already planted.

What Iowa State says to put over the plant

The home-garden page is specific about materials and about the mistake that makes them fail. Potential coverings include sheets, blankets, towels, tarps, frost fabric, or row-cover material. They work by trapping radiant heat from the ground so frost is less likely to form on the leaves, and by reducing the chance of freezing. The cover has to be held off the foliage. The page lists stakes, posts, PVC pipes, patio furniture, sawhorses, and wire loops as ways to do that. A sheet lying on a tomato conducts the cold into the leaf it is supposed to protect. Remove the covering the next day when temperatures get above freezing.

The same page gives the limit of the method. Once temperatures drop below about 28°F, covering is typically no longer effective at preventing cold damage. Damage can also show up above 28°F when the below-freezing period is prolonged, which the page describes as 5 hours or more. That 28°F line sits next to the National Weather Service glossary’s hard freeze, which is 28°F or lower for an extended period, and next to local warning criteria that are close but not identical. The frost, freeze, and hard freeze guide keeps those definitions apart. For the cloth on the bed, Iowa State’s own sentence is the one to follow: around 28°F and below, do not count on a sheet.

A fall page from the same extension program says blankets or sheets can be placed over the crop with support so they do not cause physical damage, that milk jugs or floating row covers can also be used, and that the protection should come off as soon as the temperature rises above 32°F. The two pages agree on support and on removal. They do not agree on a single trigger temperature for removal, 32°F versus “above freezing,” which is the same threshold stated two ways. Neither page gives a number of degrees of protection for a sheet.

Taking the cover off

The home-garden page says cold frames, grow tunnels, and floating row covers should be opened or removed during the day, then pulled back before temperatures drop below freezing. A cover left on through a bright afternoon is a different problem from a cover left off through a freeze. Both instructions are on that page.

What these pages do not claim

They do not say water-filled jugs, Christmas lights, or a sprinkler are required. Iowa State’s fall note mentions milk jugs as a cover option, meaning a jug used as a cloche, not a prescription for irrigation. Sprinkler frost protection is a commercial technique with its own failure mode if the ice-making stops too soon. It is not in the Iowa State pages cited here, so it is not a recommendation of this guide.

They do not say every perennial needs a cover. The home-garden page says woody plants that have entered dormancy, and plants that tolerate temperatures below freezing, are not the target of an early-frost routine. The target is the plant that will be damaged, which for a vegetable bed is the crop Table 8 put after the frost-free date, plus any plant already in flower or fruit that a crop page has flagged as sensitive.

They do not replace the decision to wait. The cheapest protection for a tomato is still the outdoor window: 1 to 8 weeks after the frost-free date in the Texas table, applied on this site to the median last 32°F date, with the 10% date left on the page so a cautious planting can use April rather than the week after the median. A cover is for the night that arrives after you have already set the plant out, or for a fall crop you are trying to hold. It is not a way to move a Texas tomato into January and call the sources satisfied.

If the forecast low is in the mid-30s and you have peppers or tomatoes in the ground, the Iowa State home page says act: cover, or move the container inside. Hold the cover off the leaves. Take it off when the temperature is back above freezing. If the forecast is below about 28°F, believe the sentence that says the cover usually will not be enough, and harvest what you can or bring the container in. Check the office that issues your warning before you translate “hard freeze” into a number. Then look at the 28°F row for your station if you want the climate context for how often a night that cold still happens this late. The row is a history. The warning is the night.

Sources

• Aaron Steil, “How to Protect Plants from Frost and Freeze,” Iowa State University Extension and Outreach, Yard and Garden: https://yardandgarden.extension.iastate.edu/how-to/how-protect-plants-frost-and-freeze
• “Fall Planting of Vegetables,” Iowa State University Extension and Outreach, Yard and Garden, frost-protection section: https://yardandgarden.extension.iastate.edu/how-to/fall-planting-vegetables
• “Growing Peas in Iowa,” Iowa State University Extension and Outreach: https://yardandgarden.extension.iastate.edu/how-to/growing-peas-iowa
• Joseph Masabni, Texas Home Vegetable Gardening Guide, Table 8, Texas A&M AgriLife Extension: https://agrilifeextension.tamu.edu/library/gardening/texas-home-vegetable-gardening-guide/
• “Guide to Vegetable Gardening on the Texas High Plains,” Texas A&M AgriLife, soil-temperature paragraph: https://legacy.research.agrilife.org/wp-content/uploads/sites/3/2011/10/UrbanVegetableGardeningTexasHighPlains_1.pdf
• NOAA NCEI station USW00013904, for the March 12 and April 2 dates used as the example.