Cool-season and warm-season crops

Two lists from Penn State

Penn State Extension’s article “Cool-season vs. Warm-season Vegetables” splits the garden by when the crop is planted relative to the last frost, and by whether the plant needs the cold. Cool-season crops are the first planting of the year. The article says that can be anytime from several weeks to a couple of months before the last frost date. Those vegetables withstand cold, and they need it to germinate, grow, set fruit, and mature. The toughest of them, which the article calls winter-hardy and illustrates with kale and Brussels sprouts, benefit from a light frost because the frost converts starches into sugar and improves the taste. The article’s list of common cool-season vegetables is: asparagus, beets, broccoli, Brussels sprouts, chives, cabbage, carrots, cauliflower, Swiss chard, kale, leek, lettuce, onion, parsnips, peas, radishes, spinach, and turnips.

Warm-season crops need higher soil and air temperatures and, in the article’s words, are always planted after the last frost date. In Pennsylvania they are usually started indoors and transplanted once the soil and the air are warm enough, because the growing season is too short for some of them to be sown directly. They have one cycle, from late spring after the last frost until late summer, when nights cool even if days still feel warm. The article’s list of common warm-season vegetables is: beans, corn, cucumbers, eggplant, melons, peppers, zucchini and summer squash, pumpkin and winter squash, sweet potato, tomato, and watermelon.

A second Penn State article, on season extenders and fall vegetables, repeats the idea and adds Asian greens to the cool-season examples. It describes warm-season crops as completing their life cycle in one growing season, often transplanted into warm soil and sometimes direct-seeded, with examples that include tomatoes, eggplants, beans, cucumbers, peppers, sweet potatoes, watermelon, and summer squash. Where the two articles overlap, the crops agree. This guide treats the first article’s “common” lists as the checklist, and mentions the second only for the Asian-greens addition and the fall use of the same cool-season group.

What those lists do not say

They do not give a ZIP code a planting day. “Several weeks to a couple of months before the last frost” is a range, and “last frost” is not tied to a 10%, 50%, or 90% probability. On this site, a week offset from a cited guide is applied to the median last 32°F date, and the page says so. Penn State’s phrase is wider than a single offset. Pennsylvania crop pages use the windows from the Penn State guide cited on those pages. A sentence without a start and an end is quoted as written.

They do say what happens when a crop is kept past its weather. Cool-season vegetables may still grow in heat, and the quality drops. Radishes turn fibrous and sharp. Warm-season crops fail in the other direction: cucumbers turn bitter when the nights are too cold. Those are quality failures, not a second frost date. A cucumber that has passed the last 32°F probability can still be bitter on a run of cold nights that never reach 32°F. The frost table will not show that. The article is the source for it.

A third Penn State page, on starting the summer garden from seed or transplants, adds temperatures that apply to “much of Pennsylvania,” which is the phrase the page uses. Peas germinate when the soil is above 40°F and grow best around 55°F to 65°F. They stop setting blossoms, which become the peas, when the temperature is above 85°F, so the page says to plant them 50 to 70 days before hot summer temperatures arrive. Folklore about sowing peas on St. Patrick’s Day is reported as folklore, not as a station normal. Root crops, including beets, carrots, parsnips, radishes, rutabagas, and turnips, should be direct-seeded because transplanting makes misshapen roots. Beets are the possible exception if they are moved carefully into loose soil. Eggplant, peppers, and tomatoes are transplanted when the soil is warm. The indicator given is air about 60°F at night and 70°F during the day, late May to early June in much of Pennsylvania. Corn may be sown from May 1 to July 1, and later plantings mature in less time. Those dates are Pennsylvania guidance from that page. They are not copied onto a Texas or California city page.

How Texas A&M’s week table sits next to the lists

Joseph Masabni’s Table 8 does not use the headings “cool-season” and “warm-season.” It uses weeks before or after the frost-free date. Laid beside Penn State’s lists, most of the warm-season crops are on the “after” side and most of the cool-season crops are on the “before” side. Tomatoes, peppers, eggplant, sweet corn, cucumbers, summer and winter squash, pumpkins, watermelons, beans, and sweet potatoes are all timed after the frost-free date. Beets, broccoli, Brussels sprouts, cabbage, carrots, cauliflower’s fall cell, Swiss chard, kale and collards, lettuce’s early edge, onions, English peas, radishes’ early edge, spinach, and turnips are timed before it, or across it.

The mismatches are the reason not to relabel Table 8 with Penn State’s headings. Mustard is 1 to 6 weeks after the frost-free date in Table 8, a warm-season position, and mustard is a leafy green that Penn State’s common lists do not even include. Lettuce is “6 weeks before or 2 weeks after,” so it straddles the frost-free date instead of sitting neatly in the cool-season bucket. Radishes run from 6 weeks before to 4 weeks after. Irish potatoes are 4 to 6 weeks before, and potatoes are not on Penn State’s warm-season list above. Cauliflower is “not recommended” in spring and is a fall crop of 10 to 16 weeks before the first freeze. A crop page follows the cell that was published for that state. A Pennsylvania reader using Penn State’s list, and a Texas reader using Table 8, can both be right about their own document and wrong about the other state’s garden.

At Austin-Camp Mabry, the station on Austin’s city page, the median last 32°F date is February 20 and the median first 32°F date is December 3. A Penn State sentence that says “a couple of months before the last frost” would point at late December if someone applied it mechanically to February 20. Table 8 does not say a couple of months. Its early cool-season offsets are on the order of 4 to 10 weeks, depending on the crop. Onion plants are 4 to 10 weeks before. Spinach is 1 to 8 weeks before. The Texas cell is the one the Texas page uses. The phrase “a couple of months” stays in the Pennsylvania article.

The first-freeze side is how cool-season crops return. Penn State’s fall article says cool-season vegetables are also the ones planted later, which is why they are called shoulder or fall crops, and that they are the crops season extenders are built for. Table 8 already has the fall column: spinach 2 to 16 weeks before the first freeze, lettuce 10 to 14 weeks before, kale and collards 8 to 12 weeks before, radishes 1 to 8 weeks before. From Camp Mabry’s December 3 median, “2 weeks before” is November 19 and “16 weeks before” is August 13. That is the width of the spinach cell, not a suggestion to sow spinach every week from August to November. Heat in August is a separate problem the cell does not solve. The cell only places the crop relative to the freeze.

Using the checker without collapsing the categories

The “is it inside a published window?” check does not ask whether a crop is cool-season or warm-season. It asks whether today’s date falls inside a window that has two endpoints from a cited guide. A tomato on February 21 in Austin, against Camp Mabry’s February 20 median, is still before the “1 week after” edge of February 27, so the checker should say it is outside the Texas window. March 1 is inside that window, which runs from February 27 through April 16. The plant’s membership in Penn State’s warm-season list is not what the checker computed. A spinach sowing in the middle of its before-frost window is inside because of Table 8, not because spinach appears on a cool-season list.

If your state has no cited guide, the station’s 32°F probabilities are still shown. A Penn State window stays a Pennsylvania window. The lists here say which crops are usually discussed as cold-tolerant. A local extension date is what makes a day an extension date. The succession guide is the practical follow-on: cool-season crops are the ones WVU says to pull and replace with warm-season crops, and the replacement still has to land inside the warm-season crop’s own window.

Frost at 36°F, 32°F, and 28°F remains a different question from “cool” and “warm.” Kale benefiting from a light frost, in Penn State’s wording, is not a claim that kale is unharmed at 28°F for five hours. Iowa State’s cover guidance, in the late-frost guide, is the source for that severity. The category tells you which side of the last-frost date the crop usually belongs on. The station table tells you when 32°F occurred in the climate record. The forecast tells you about the night ahead.

Sources

• Penn State Extension, “Cool-season vs. Warm-season Vegetables”: https://extension.psu.edu/cool-season-vs-warm-season-vegetables
• Penn State Extension, “Season Extenders and Growing Fall Vegetables”: https://extension.psu.edu/season-extenders-and-growing-fall-vegetables
• Penn State Extension, “Starting Your Summer Vegetable Garden—Seeds or Transplants?”: https://extension.psu.edu/starting-your-summer-vegetable-garden-seeds-or-transplants
• Joseph Masabni, Texas Home Vegetable Gardening Guide, Table 8: https://agrilifeextension.tamu.edu/library/gardening/texas-home-vegetable-gardening-guide/
• NOAA NCEI station USW00013958, Austin-Camp Mabry, for the February 20 and December 3 medians.