Our Local Climate Has Changed

USDA Plant Hardiness Zone Map, 2023. Agricultural Research Service, U.S. Department of Agriculture. Accessed from https://planthardiness.ars.usda.gov/.  Click the link for access to state and regional interactive maps.

I’m prompted to write this post to alert growers to the publication of the revised USDA Plant Hardiness Zone Map (often incorrectly called the Cold-Hardiness Zone Map). Canada and Mexico are now included. Most areas are a quarter-zone warmer in winter than in 2012.

Click this link for more on the history of the USDA Plant Hardiness Map. Note that the original intention of this map, which shows the average annual minimum winter temperature, was to indicate which perennial plants (such as fruit trees or shrubs) would survive the winter in each location. It does also indicate which biennial and annual plants can successfully overwinter (on average). There are lots of things it was never designed for, such as indicating how long the winter lasts, or how many days will be below a certain temperature. It is very valuable for its purpose: showing which plants will survive in an average winter.

Zones 10 and 11 were added in 2012, and Zones 12 and 13 are added to the new 2023 map. Zones 12 and 13 are presently only found in Hawaii and Puerto Rico, where the average annual minimum temperatures are above 50°F (10°C) and 60°F (15°C) respectively. The new interactive map has closer definition between neighboring areas than previously, and can be examined at a finer scale. A city or town with heat-absorbing concrete, can be seen as warmer than surrounding countryside.

Comparison of plant hardiness zones mapped for the 11 largest Metropolitan Statistical Areas [MSAs (U.S. Census Bureau, 2003)] in the 1960, 1990, and recently revised Plant Hardiness Zone Maps.

This technical article by Widrlechner and others in HortTechnology in 2012 gives a short history of the Map, and explains how it can be used to predict winter-injury to plants, and avoid damage by changing shipment dates or adding plant protection. It explains how freeze injury can occur during three stages in the annual cycle of plants: firstly, extreme cold occurring during the fall, before plants have adapted to such temperature levels; secondly, during mid-winter when extreme cold can still overwhelm the plants’ adaptive survival mechanisms; thirdly, during late winter and early spring of plants “deharden” after satisfying their minimum physiological rest requirements, in unseasonably warm spells.

The article also discusses attempts in various regions to add in the effects of other weather features such as cloud cover, wind speed and snow cover. It also discusses the 1997 AHS heat zone map, and shows graphs correlating the Plant Hardiness Zone Map and the Heat Zone Map for three parts of the US. Note the date of publication: references to the “updated” map refer to the 2012 version, not 2023.

Pablo Batavian lettuce. The batavians are our most bolt-resistant heat-tolerant lettuces.
Photo Nina Gentle

For summer weather, the American Horticultural Society in 1997 created a Heat Zone Map to show how many summer days are likely to exceed a certain temperature. It is not the mirror image of the USDA Plant Hardiness Map – it does not show the yearly average hottest temperature. I think an update of a heat zone map is due!

AHS Heat Zone Map 1997

According to the AHS map, I am in Zone 7 for summer temperatures (61-90 days a year reaching 86°F (30°C) or more) — right in the middle of the range. According to Weatherspark.com, from June 11 to August 30, our average daily high temperature is 86°F (30°C) or more. Summer nighttime temperatures are also an important feature of the weather, for us and our crops.

Twin Oaks is in USDA Plant Hardiness Zone 7a. (our zone remains unchanged after the November 2023 update, but we did move 5 Fahrenheit degrees milder from our 1990’s 6b to 7a in the 2012 revision). The average annual minimum winter temperature in zone 7a is 0°F–5°F (–18°C to –15°C). I have noticed that our local climate has changed on average, over the past ten-twenty years, to become drier, with milder winters and hotter summer nights.

Savoy cabbage with frost.
Photo Lori Katz

Here are other features of our weather at Twin Oaks, Louisa, central Virginia:

  • Our average rainfall for a year is 37″ (100 cm), fairly evenly distributed throughout the year, at 2.2″–3.6″(5.6-9.1 cm) per month. February is the driest, May the wettest.
  • Our average daily maximum temperatures are 49°F (9.4°C) in December and January, 89°F (31.7°C) in July. The average night low temperatures are 29°F (–1.7°C) in January, 69°F (20.5°C) in July.
  • Our season from last frost to first frost is around 211 days. The average date of the last spring frost is April 24 (later than May 7 only happens one year in ten); the average date of the first fall frost is Oct 14 (earlier than Oct 1 only happens one year in ten).

    Such grey weather! But will it rain?
    Photo Wren Vile
  • Our climate is controlled by three weather systems, mainly by moisture from the Gulf of Mexico, but also by the Bermuda High Pressure area in summer and the recurrent waves of cold Canadian air in winter. Due to the erratic movement of thunderstorms, some parts of our area may experience long periods of drought. September–November is the dry season but also the hurricane season.
  • Our latitude is 38° N, which is very relevant to onion growing and to daylight length.
  • Our period when daylight length is less than ten hours and little plant growth occurs, lasts from Nov 21 to Jan 21.
Climate Summary from Weatherspark.com

To learn some of your other weather details, see Weatherspark.com.

Cover Crops for November: Winter Rye and Austrian Winter Peas

Cover Crops for November: Winter Rye (with Austrian Winter Peas early in November)

Winter rye headed up. Mow or turn it under very soon! Don’t let it shed seed.
Photo Pam Dawling

The first half of November is the last chance to sow winter cover crops in central Virginia

Our average first frost is October 14-20. If yours is later, see my post Cover Crops for October. It is still worthwhile to sow a few cover crops up to three weeks past your average first frost, and I’ll tell you about those, with ideas on what to do if your climate is past that point.

If the area is ready for cover crops up to 10 days past the frost date, sow winter wheat or winter rye and hairy vetch or Austrian winter peas.

See Planning Winter Cover Crops, a post that includes my Short Simple Guide to Winter Cover Crops and my slideshow Cover Crops for Vegetable Growers.

Last Chance Cover Crops in November: Rye

Rye grows impressive roots that improve the soil. Kauffman Seeds

Winter rye can still be sown in the mid-Atlantic in the first half of November. Winter rye is the cereal grain rye, not ryegrass. Ryegrass comes in annual and perennial forms, but neither is a good winter cover crop here. They don’t produce as much biomass as cereal rye, and can become weeds in our climate.

Winter rye is hardier than any other cover crop and can take later planting dates. Some people do sow winter wheat in early November rather than winter rye, so if you don’t have rye seed, use wheat. It can be tricky to have the cover crop seed you want, without risking buying more than you need. Sometimes it pays to use what you already have, as it may not give good germination if saved over to next fall.

Winter rye has an allelopathic effect (inhibition of germination) on small seeds, that lasts three weeks or more after rye is turned under. This means you will need to be fairly confident that the weather will allow you to till the rye in with three weeks to go before sowing carrots, spinach or other spring crops with small seeds. Transplants are not affected in the same way. If necessary, reconfigure your crop rotation plan over the winter. Plan for the next food crops after winter rye to be ones planted after late April, such as late corn plantings, winter squash, transplanted watermelon, sweet potatoes, tomatoes, peppers, June-planted potatoes, fall brassicas, and second plantings of summer squash, cucumbers, beans.

A field of winter rye with a strip of crimson clover in early May.
Photo Bridget Aleshire

Other key features of winter rye cover crop

  • Rye grows 5′-7′ (1.5-2.1 m) tall.
  • Mow-kills at flowering, but not earlier.
  • Suppresses weeds (especially lambsquarters, redroot pigweed, ragweed).
  • Adds lots of organic matter if grown to full size.
  • Improves the soil’s ability to absorb and store water.
  • The roots hold the soil together and greatly reduce erosion.
  • Can be used to scavenge nutrients left over from a previous legume crop or to hold onto nutrients applied for a crop that failed.
  • Sow from 14 days before to 28 days after first fall frost.
  • Don’t sow before September in zone 7 – it may set seed.
  • Rye makes little growth in mid-winter, but very good growth once spring arrives.
  • Rye can be sown in the spring, although when incorporated, oats break down quicker.
  • Can be undersown in sweet corn or in fall brassicas in early September, and left as a winter cover crop.
Cover crop of rye and winter peas with flowering crimson clover. Photo Bridget Aleshire.

Include Austrian Winter Peas if possible in the first week of November

See my September post for more about the benefits of including legumes with winter cover crops grasses. Also how to inoculate legume seeds with the nitrogen-fixing bacteria. Austrian winter peas can be sown later than other legumes. See my October post for more about those. See Working with the time you have left for options if you are in another climate zone.

A key to success with legumes is to sow early enough in fall to establish before winter halts growth, and to plan not to need that plot next year until flowering time for that legume. Austrian winter peas bloom here at the end of April (about a week later than crimson clover, and a week earlier than hairy vetch). (If you have a legume that doesn’t reach flowering, you get less nitrogen for your money, and will need to add some compost or other source for the following food crop.) Suitable crops for following Austrian winter peas are ones planted after May 1: winter squash, melons, sweet potatoes, tomatoes, peppers, middle sowings of sweet corn, June-planted potatoes. This is pretty much the same list as crops that can be planted after winter rye.

Austrian winter peas
Photo https://www.uaex.edu

October 15 (our average frost date) is our clover/peas watershed (legume-shed?). Before that date we use crimson clover; after it (until 11/8, 3 weeks after our average frost date) we sow Austrian Winter Peas, along with winter rye or winter wheat.

Austrian winter peas winter-kill in zone 6, but are hardy in zone 7. Hardy to 0°F (-18°C). Sow Austrian winter peas at least 35 days before first hard freeze (25°F/-4°C) – in zone 7, that’s a 50% chance on 11/8. See Weatherspark.com for info on your location, or Dave’s Garden.

Last Chance cover crop

In the second week of November, we sow winter rye alone as our last chance cover crop. It is too late for any legumes.

Grain seed will store OK for the next year, but peas and beans really lose viability fast. If you still have Austrian winter pea seed from last year, I’d say throw it in this November rather than keep it for a third year, even if you are past the usual date. Germination rate goes down to 50% after a couple of years, and the plants won’t be sturdy.

Barbara Pleasant tells us two other ways cover crops can improve soil: Rhizodeposition and bio-drilling.

Rhizodeposition is a process whereby plants release sugars and other substances through their roots. The root tips host colonies of helpful microorganisms that go deeper as the roots grow deeper.  For rye, this depth can reach 6 feet (2 m)!

Bio-drilling is what happens when cover crop roots “drill” into compacted subsoil. Oilseed and daikon radishes are cover crops famous for this action. The roots push deep into tight subsoil. Bio-drilling also happens when deeply rooted cover crops penetrate the subsoil and then die. With winter rye this can happen when the headed-up rye is mowed close to the ground in spring, or the rye is tilled into the top soil, severing the roots below till level. The next crop can follow the root channels made by the cover crop. This gives access to nutrients (including dead roots and microbes) left behind by the cover crop.

How to protect the soil over the winter if it is too late for cover crops

Carrot harvest cart. Our Danvers carrots have plenty of sturdy leaves to protect the soil over winter. Photo Mari Korsbrekke

Sowing cover crops too late means you don’t get enough growth in the fall, and the soil is not adequately protected from erosion or from weed growth over the winter. Try really hard not to leave bare soil over the winter.

If it’s too late to sow cover crops, but you do have a healthy growth of weeds, mow them at the beginning of November and then leave everything alone until early spring. The weed roots will hold the soil together and the above-ground growth will protect the soil from heavy rain. Having live plants will provide food for the microbial life in the soil.

If you don’t have weeds, but only almost bare soil, and it’s too late to sow cover crops, find some kind of organic mulch to cover the soil. When we harvest our storage carrots in November, we return the cut tops to the soil surface. Another seasonal option is tree leaves (sometimes conveniently left in bags by the curb). Best to ask the “owners” before lifting them. Other ideas include straw or hay. Woodchips or sawdust will work for winter protection, but don’t till them in when spring arrives. Rake them off and compost them nearby. If turned under, they use up a lot of nitrogen decomposing, and your crops will be starved.

More resources on cover crops

My book Sustainable Market Farming has a chapter on cover crops and many pages of charts about particular options.

Cover of Managing Cover Crops Profitably book from SARE

The book Managing Cover Crops Profitably (third edition) from the Sustainable Agriculture Research & Education Program (SARE), is the best book I know on the subject. You buy the book for $19 or download it as a free PDF from SARE.

More strategies for dealing with a changing climate – article in Growing for Market

GFM-January2013-cover200px

The January issue of Growing for Market is out, and in it is my article More strategies for dealing with a changing climate. A photo of our fava beans is on the cover.  This is the third in a series of four. (You can see earlier blog posts about the first two, in the Articles category.) This article covers the use of soil temperature as a deciding factor on when to sow or plant, and includes a table of minimum (spring) and maximum (summer/fall) temperatures for about 50 crops. As the climate becomes harder to predict, using a calendar (“Plant potatoes on St Patrick’s Day!”) will need to be replaced by using information like soil temperatures, which reflect what the plants will actually experience this particular year.

The article also discusses scouting, which is the practice of making a regular tour of your crops to monitor growth and health. If you see a pest or a disease, you can determine if the level of infestation is enough to call for action, or if watchful waiting is in order. Keeping in touch with how your crops are doing will help you know when you need to take action to avert disaster or to make good use of an opportunity like an early-finishing crop opening up the possibility of using a longer-term cover crop.

droughtI also talk a bit about being prepared for more extreme temperatures – trialing varieties that are more cold- or heat-tolerant than your old favorites, and using shadecloth and organic mulches to reduce heat stress.  ATTRA’s Drought Resistant Soil addresses ways to increase the organic matter content of the soil, and keep the soil covered at all times, helping you farms’ resilience.

In addition I added in a few more resources I’ve found to help with predicting climate change. DailyClimate.org – a daily email newsletter; NOAA Climate Prediction Center, and Weatherspark.com, a fun weather site is where you can see, for instance, what your average winter low has been, and plan plantings accordingly.

Two additional resources on frost management are NCSU’s Frost/Freeze Protection for Horticultural Crops and the Food and Agriculture Organization 126-page book Frost Protection: Fundamentals, Practice and Economics.

Favas, spring sown, good germWhy the fava bean photo? Wait till the soil temperature reaches 36F (2C) before sowing.

As well as my article, there are many other gems – Identify your biggest money-making crops by Chris Blanchard; A Tool Review of The Quick Cut Greens Harvester by Jean-Martin Fortier; 8,000 miles and 18 farmers markets, a travelogue by Gwynn Hamilton and Bert Webster about their cross-country road trip visiting farmers markets all the way; Understanding one of the few insecticides for organic growers by Raymond A. Cloyd, about spinosad formulated as Entrust, and Growers create their own wholesale market for local flowers in Seattle by Debra Prinzing, co-author of The 50-Mile Bouquet  about the movement toward locally grown, sustainable flowers.