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Archive for the winter tag

USDA releases new plant hardiness zone map

December 11, 2023 • Leave a Comment

The 2023 plant hardiness zone map is based on 30-year averages (from 1991 to 2020) of the lowest annual winter temperatures at specific locations. It is divided into 10-degree Fahrenheit zones and further divided into 5-degree Fahrenheit half-zones. The 2023 map incorporates data from 13,412 weather stations compared to the 7,983 that were used for the 2012 map.

Plant hardiness zone designations represent the “average annual extreme minimum temperature” at a given location during a particular time period (30 years, in this instance). Put another way, the designations do not reflect the coldest it has ever been or ever will be at a specific location, but simply the average lowest winter temperature for the location over a specified time. Low temperature during the winter is a crucial factor in the survival of plants.

As with the 2012 map, the new version has 13 zones across the United States and its territories. Each zone is broken into half zones, designated as “A” and “B.” For example, zone 6 is divided into 6a and 6b half zones. When compared to the 2012 map, the 2023 version reveals that about half of the country shifted to the next warmer half zone, and the other half of the country remained in the same half zone. That shift to the next warmer half zone means those areas warmed somewhere in the range of 0 to 5 degrees Fahrenheit. However, some locations experienced warming in the range of 0 to 5 degrees Fahrenheit without moving to another half zone.

The annual extreme minimum temperature represents the coldest night of the year, which can be highly variable from year to year, depending on local weather patterns. Some changes in zonal boundaries are also the result of using increasingly sophisticated mapping methods and the inclusion of data from more weather stations.

Temperature updates to plant hardiness zones are not necessarily reflective of global climate change because of the highly variable nature of the extreme minimum temperature of the year, as well as the use of increasingly sophisticated mapping methods and the inclusion of data from more weather stations. Consequently, map developers involved in the project cautioned against attributing temperature updates made to some zones as reliable and accurate indicators of global climate change (which is usually based on trends in overall average temperatures recorded over long time periods).

https://planthardiness.ars.usda.gov/

ELSEWHERE: How plants survive a Michigan winter

Your plants survived winter—only to be snuffed by spring?

March 21, 2022 • Leave a Comment

How to help your plants make it through the frosty weeks of April

by Janet Macunovich
Photographs by Steven Nikkila

Plants are most susceptible to cold after they begin to wake up in spring. Buds, bark and roots that were hardened to the point of being able to weather minus 20 degrees in January become irreversibly soft by late March or early April. Then they may be seriously damaged at 25 degrees and killed outright at 10 degrees. Sudden, large drops in temperature hurt them most, and that’s what April frosts usually are—frigid packets of air that drop from clear skies after a balmy, slightly breezy day goes still at sunset.

A magnolia hardens its tissues in late fall and winters its flowers under fuzzy bud caps capable of withstanding minus 15 degrees. Yet once the sap begins to flow in the tree and the bud caps split open, the flowers may be damaged by frost. It’s not practical for home gardeners to protect trees from frost, but this article has tips on how to help other plants past this most treacherous of times.

**Frost damaged some petals on this saucer magnolia (*Magnolia x soulangiana*). The damage won’t hurt the tree, but it can spoil its show.**

Consequences of spring freezes may take days or even weeks to develop and are often lumped in with “winter kill.” We may even fail to recognize them as cold damage because they appear long after we’ve forgotten the frost. Then we see leaf drop, bud blast, aborted flowers, splitting bark, scorched foliage, freeze-dried twigs, stunted growth, susceptibility to disease, and failure to flower or flourish.

We often spend hours in late fall putting special plants to bed with wilt-stop coatings, burlap screens, elaborate mulch blankets and one last watering before the ground freezes. Those measures may have made a difference—that marginally hardy rosemary may have survived in its protected alcove and the rhododendrons and boxwoods stayed moist enough to bring their leaves into April unscathed. Yet what’s alive right now may be relegated to the compost pile and labeled “not hardy” if we let down our guard during this last leg of the journey.

Here are ideas to help your plants pass without harm through these next few, most risky weeks.

**Oakleaf hydrangea (*H. quercifolia*) is another plant that must carry its tip buds from one summer right into the next in order to bloom.**

Once that densely coated bud cover opens, I try to protect the emerging hydrangea shoot from freezes. If that bud dies, I’ll certainly see the plant’s pretty foliage this year but probably won’t see any bloom.

Moist soil makes a warm night.

You can fight frost with water, but you’ll have to aim the hose at the plants’ roots, not up at the leaves and stems.

An inclination to sprinkle at-risk foliage is understandable if you have seen nursery stock or an orchard protected from spring frost in rather dramatic fashion, by sprinklers that run all night to coat the plants with ice and all morning to ease the ice’s passage. However, this technique is one that warrants a “don’t try this at home” caption. That’s because it’s the creation of ice, not its simple presence, that works the magic. Plants benefit from the heat given off by each new drop of water as it enters an icy state. Standard oscillating and rotating sprinklers can’t deliver the amount of water or relentless coverage needed to foster continual ice formation.

What you can do with water is to stoke the radiator below each plant. Keep an eye on the three-day forecast so you can tend your plants the day before a frost is due. Soak the soil—avoid splashing the branches and foliage. Moist soil absorbs more heat from the air by day and radiates it longer and more steadily into the night than dry soil. That’s one reason that frost is always more likely over sandy ground than clay.

Don’t over-do. If your garden is very well-drained, you may be able to water with abandon but if there’s any chance the soil will become waterlogged, quit. Soggy roots are more trouble to a plant than frosted tips.

Everything’s warmer under a tent.

Everyone knows that heat rises but many are surprised to know just how much warmth can radiate from the soil. Soil temperature in the Midwest in late winter and early spring may be in the 20s or perhaps 30 degrees from ground level to two feet deep. Below that the ground remains at about 50 degrees. If the soil is loose and airy, that heat rises steadily through and out of the soil. It can preserve plants and plant parts close to the ground during a frost.

Cover a plant with something that seals its connection with this radiator. Within that tent the air may be 5 degrees warmer than in the open.

Fiber makes a great frost blanket.

Don’t use plastic to cover plants, or if you do, place props to hold it well up off the leaves and twigs so it can trap ground-warmed air around all parts of the plant. Since plastic is such a poor insulator, any bit of plant touching the plastic is essentially in contact with the frosty air and likely to freeze as if unprotected.

The better choice for frost protection is cloth, replete with tiny air pockets. You can use old linens and blankets if they’re light enough to cover without crushing delicate foliage. To use heavier materials, first place props to bear the weight.

Lightweight fabrics developed specifically for plant protection are called floating row cover, frost blanket, frost cloth, plant cover, and spun-bond polypropylene—all with various brand names. These products are light enough in most cases to be supported by the plant itself and come in sections or rolls wide enough to cover a shrub or a whole bed of flowers or vegetables. Permeable to light and water, they can be left in place even on cold days without smothering the plant they’re protecting. They are made in a range of weights from about one-half to several ounces per square yard. In general, the heavier the cloth the more light it blocks, but the more heat it can hold—the lightest promise just 2 or 3 degrees of frost protection while the heaviest claim eight.

Floating row cover protects newly planted bear’s breeches (*Acanthus mollis*) during a frost. Lenten roses (*Helleborus x orientalis*) in the foreground require no protection from this frost because they’ve developed in place and are in sync with normal weather. The newcomer has just come from a greenhouse and is beyond the stage of growth it would be if it had grown on-site.

**Frost protection should be removed once danger of frost has passed, to avoid overheating covered plants.**

It’s a cloche call.

The traditional cloche is simply a large glass jar that can be inverted over a plant. More recently, sections of glass or plastic clipped together to make tents over individual plants or rows have been included under this term.

Plastic tunnels covering rows of plants are sometimes called cloches, too. They’re made by bending heavy wire into big U shapes, sticking them upside down into the ground, and then covering them with plastic from a roll. Glass is better than plastic in terms of the light, warmth and humidity it affords a plant, but shatterproof plastic is safer for garden use. Plastic cloches can provide protection from hail as well as frost.

Plastic milk jugs and soft drink bottles can be pressed into service as cloches. Cut off the bottom of the jug or bottle and press it firmly into place as you cover the plant so its cut edges sit securely in the soil. If days are warm, remove the bottle cap to vent excess heat.

A modern twist on the traditional cloche are season extenders like the Wall O’ Water. A plant is encircled by this cylinder of compartmented plastic—basically a double sheet of heavy plastic seamed to form pockets that can be filled with water. The water absorbs radiant energy by day and at night releases it to the plant in its embrace.

If you have special small plants and an abundance of two-liter soft drink bottles, you can make your own wall of water. Cut the bottom off one bottle and circle that container with six intact bottles. Use duct tape to hold the group together. Slip the center bottle over a plant. Remove that bottle cap if days are warm, to prevent heat build-up. Fill the surrounding bottles with water.

Gracious sakes, come in outta that wind.

Our mothers, grandmas and aunties were right—one can catch one’s death in a wind. Cold that wouldn’t otherwise harm a plant can kill if drying winds accompany it. Wind is even more damaging to plants during early spring cold spells than it is during the depths of winter. So keep that burlap screen or snow fencing in its place several feet to the windward side of your broadleaf evergreens.

One reason spring wind is so devastating is that air temperatures during the day are warm enough that a holly, rhododendron, boxwood, pieris or grapeholly leaf can sustain photosynthesis. Those leaves lose water through their pores during that process. Yet a cold snap can freeze the top inch of soil where these plants have many fine roots. The next day, the plant sits in the sun losing water through its leaves but the water the roots need to replace it is locked in ice. When water runs short, the losers are the aerial parts furthest out—tips and emerging buds—as well as leaf edges that have “softened” as they quit the devices that added up to seasonal hardiness. We often don’t see the damage until May, but the scorched tips and leaf margins we see at that time began in that April freeze.

If you wonder whether you should protect plants from wind, look around your neighborhood for trees that stand alone, away from the protection of a grove or any buildings. Are those trees symmetrical or distorted? Distortion on one side can indicate severity and direction of the prevailing winds that dry and kill in late spring.

Cover those feet.

Mulch is a another strategy that’s important to understory species with shallow roots, such as rhododendron, holly, Japanese maple and magnolia. A study in Wisconsin put a number on the value of mulch. That is, when air temperature falls to 1 degree, the soil there drops to 16 degrees if it has a three-inch mulch blanket, but can stay at about 22 degrees given six inches of cover.

If you begin to rake off special plants’ extra mulch on warm spring days, don’t be too quick to cart it away. A friend has recently recommended sawdust as a material that can be easily peeled back yet replaced in a hurry. I intend to keep some on hand to rectify the silliness of Japanese wax bell (Kirengeshoma palmata), which is forever leaping up too soon and being cut down by frost.

What gardeners do best: Improvise.

I don’t have many things that I keep on hand especially for frost protection. Like most gardeners, I improvise. Upended plastic garbage pails, five gallon buckets, milk crates, old draperies and cardboard boxes are all in my April bag of tricks. I’ve also grinned over the use of beach umbrellas in one friend’s garden and discussed with another how we might use a hunter’s open-bottom ground blind to protect shrubs and even small trees.

Thank heaven for plants that just take what Nature hands them and keep on chugging. These Red and Yellow Emperor tulips have reckoned with snow and frosts for 20 years, even freezing solid in a late April snow.

Here they are just one day after appearing in the previous, snowy scene. So long as I don’t walk on them while they’re frozen—an abuse resented by all plants from lawn grass to wisteria vines—they come back with gusto.

Dream of ideal plants and places.

Every spring I vow to eliminate all plants that cause me extra work. Every summer at least a few of them convince me anew that they’re worth the effort. Thank goodness for the existence of perfect plants—species that can handle whatever “normal” happens to be each spring. I’m grateful, too, for ideal places, such as that wind-protected spot about ten feet southeast of my neighbor’s six-foot hedge. It is ground that happens to slope to the southeast so it sheds cold air downhill and catches warming sun early in the year. It’s sandy, well-drained soil there, so it conducts ground warmth well and doesn’t add oxygen deprivation to a root’s winter woes. I can’t ask for more protection.

But I can ask why the spot’s not bigger. I’d like to put my whole garden into it.

Article by Janet Macunovich and photos by Steven Nikkila, www.gardenatoz.com.

Garden Forecast: Pleasantly Foggy

March 29, 2017 • Leave a Comment

After winter, predicting what happens to garden plants is as tricky as predicting the weather or the economy

Even a leafless tree can provide a windbreak and hold in a bit of radiant heat from the ground during the coldest winter nights, all good things for a plant at its feet. Yet none of this is any consolation if an ice-laden limb of the tree flattens everything in the understory!

Perhaps that mountain ash, burning bush, lilac or rhododendron died as a result of black walnut blight, poisoned by accumulation of chemicals produced by walnut roots. Or maybe the victim would have survived with more moisture. The more water that goes into the soil under a black walnut, the more the toxins are diluted, so that even sensitive plants may survive.

This past winter was odd, weather-wise. That’s not surprising, since few seasons ever match all the averages. At winter’s end the standard greeting between two gardeners is “Hello! Weird winter, huh? What do you suppose all that (fill in weather feature here) will mean to the garden this year?”

Needled, waxy species such as junipers usually have an advantage over broadleaf evergreens like boxwood and rhododendron in an exposed and windy winter. Yet if this big plant was newly planted last fall, still possessing only the unnaturally small rootball of the nursery field, wind may have rocked the plant and broken many roots.

Have fun soliciting and making predictions but treat it as what it is, a game. Use it to pass the time until spring arrives with the real answers. Don’t take any prediction too seriously, no matter the source.

A warm winter may be easier than usual on hybrid roses. But single-digit air that arrives in late winter when there is no insulating snow over the roses’ roots may do more damage than another year’s sub-zero snowy January.

That goes for the pictorial prognostications that accompany this story. Don’t fret over any of them, just consider them as the possibilities they are.

My goal here is to convince you that it’s not possible to make an accurate forecast of what effect a particular stretch of weather will have on a given garden. It’s far better, and more fun, to match plants and sites, then see what happens as millions of years of genetic development goes head to head with all the vagaries of the rest of the natural world.

If you want to explore this position, start by comparing garden forecasting with two disciplines that rely on prediction: meteorology and economics.

First, there’s meteorology, a science that even the most critical person admits has improved over the past 40 years in its ability to predict tomorrow’s weather. To make predictions, forecasters use precise data collected from thousands of weather stations across and above the world. These numbers are transmitted instantaneously to central reporting offices where all the factors that influence the speed, direction, temperature and humidity of air currents, plus the atmosphere’s current vital signs, are fed into sophisticated computers. There they churn as hundreds or thousands of equations whose answers are compared to known history and probability, then displayed as predicted future air pressure, wind speed, cloud development, etc.

This system for analyzing the atmosphere had its beginnings in prehistory when the first farmer or sailor squinted into the wind and tried to recall when he or she had seen a sky quite like that and what had followed in that sky’s wake. By 2,000 years ago, these forecasters had help from weather vanes and rain gauges, but it wasn’t until the mid-1600s that they understood the need to measure air pressure, humidity and temperature and invented the barometer, hygrometer and thermometer. Thus today’s computers have no more than 350 years of data to work with. For many New World areas the records cover less than 100 years.

Beautiful but stupid, that’s a forsythia. Just a few days of cold then a warm spell can trick the chemical clock in a forsythia bud into acting like winter is over. Flowers that open early, even part-way, won’t be part of the spring show. Some other plants that bloom out of season in late fall, such as azaleas and lilacs, may fare better next time if fertilized differently. A nutrient-deficient bud may not harden as well or set as dependable a chemical clock.

With simple math (at least I’m told it’s simple!) one can look at the number of variables, the amount of historical data on hand, the possible combinations of variables and locations for which we desire weather forecasts and see that we just haven’t been at this game long enough to know all the possible answers.

Lilac is a tough character that isn’t phased by unusual winter temperatures. Yet, if a foot path to the school bus stop passes over this plant’s roots, it may be in trouble. That pressure on the soil, buffered during a normal winter by ice in the ground, may pack the soil all year if the soil is not frozen. Soil there may now be so compacted it’s suffocating the roots.

Another place where accurate forecasts would be gold—literally—is the field of economics. It’s a science barely 300 years old and which only took on its current form about 70 years ago. It’s so new that we really don’t expect reliable predictions and accept widely varying interpretations of the same “leading indicators.” Economists are still debating how much influence each accepted variable has on overall economic growth and recession, and theories are still being advanced and tested that would change the variables themselves.

So there you have it, two areas where there is a pressing need for accurate forecasting. In one, we’ve strived for thousands of years yet we’re still only close in our predictions. In the other, although the search for reliable forecasts is fueled by the weight of all the world’s money and we have hair-splittingly accurate accounts of every conceivable factor for 70 years, experts still can’t agree that we’re even looking at the right numbers.

This coral bells might have gotten ahead, making more roots than usual during a warm winter. However, if there is no snow to insulate it against dry, cold winds in late winter, it may lose as much leaf to dehydration as the extra starch in the roots will be able to replace.

The basic natural factors that affect an individual plant’s performance are at least as complex as those that influence the weather. And because people are the ones who plant, prod and rate the plants whose futures we’d like to predict, human actions have to be taken into account, too.

Just to start building a history on which garden predictions might someday be based, we would need complete meteorological records for the garden area plus accurate daily measurements and seasonal averages of soil density, temperature, moisture levels, available nutrients and resident pathogens. Also important would be an objective evaluation of the relationship—beneficial or antagonistic—between each pair of plants so we could weight a plant’s possible response to adverse conditions for whether it was being assisted or debilitated by its neighbors. Of course we’d need reports on all human, animal or insect activity in the vicinity and detailed descriptions of the plants themselves, including their ages and past “medical” history.

Plants such as astilbe (also coral bells, rhododendron, azalea, yew, burning bush and many more) suffer extra damage from root loss if root-chewing black vine weevils were living in the soil near their roots in winter. The weevil grubs feed every day that the soil is not frozen, even under January’s and February’s snow. But if moles and shrews are also active during a warm winter, the weevil grubs may be wiped out.

Meteorologists turn to the National Weather Service for reports. Economists tap the National Bureau of Economic Research for essential statistics. Pressing need and the importance of money fuel these data-gathering efforts. There’s no big pay-off in collecting garden stats. So don’t hold your breath waiting for the Garden Prognostication Agency to appear.

Do keep doing your best to match each plant you grow to a site that provides the conditions it would have had in its native setting. Embedded in every well-sited plant is the ability to survive just about everything that Nature can throw at it. Grow it in the right amount of light and in the type of soil that species evolved to exploit. Water it as if you are the gentlest rains of that plant’s homeland. That plant will not only light up your life in a “normal” year but provide you with something to crow about in the bad times.