Frederick Dobbs On The Sub-Zero Garden

The dog didn’t seem to be cold. I wasn’t either, but it was no warmer than —15°F. It was a clear, still night; we had gone out for a walk before bed and nothing much seemed to be moving. The only sound I was aware of was the brittle, dry snow crunching beneath our feet. As we came to the corner, under the street light, I saw a puddle of water, unfrozen. I wondered at it and reflexively kicked a dusting of snow into it. It froze almost instantaneously from the points of contact with the snow.

Why the puddle was unfrozen in the first place I don’t know. The temperature had been well below freezing for a week or more, so the puddle must have existed for that long anyway. On first glance, freezing water seems like the simplest thing in the world. But a closer look exposes more complexity. The same is true with the interaction of plants and freezing temperatures.

During the course of the year, plants from temperate regions change in their capacity to tolerate and survive freezing temperatures. Very few herbaceous plants can tolerate low temperatures for long; some have little or no tolerance at all. But after a period of cold acclimation, some perennial species are extraordinarily tolerant. Trees native to the boreal zone withstand temperatures of –40°F for months and in midwinter survive temperatures lower than that. But even species that are quite cold hardy in midwinter can be damaged by frost during their growing season.

Everybody knows that ice melts above 32°F. What’s less well known is that pure water (not rain water or tap water) is unlikely to freeze at temperatures much warmer than —40°F; the attribute of water to remain liquid at temperatures well below the freezing point is called “supercooling”. When water freezes, it becomes crystalline, but this transition does not usually occur spontaneously at temperatures warmer than —40°F. A germ or seed that initiates or “nucleates” ice crystal formation and from which crystals grow is required. This is how the snow triggered the puddle to freeze on my walk.

“Ice nucleators” are important, if not essential, in the formation of snow and rain, and they are ubiquitous in the atmosphere. Most are bacteria; a diverse range of bacterial species presumably deposited by snow can be isolated from high mountain peaks that would otherwise seem sterile. Air-borne dust also plays a role in ice nucleation, and some plant constituents are ice nucleators. Water in nature has an abundance of ice nucleators, and so it generally freezes at or slightly below 32°F.

Most plants, even very tender ones, have some tolerance for cold temperatures, and for most, frost during their growing season is a real possibility. Think of northern orchard crops, apples or cherries, whose flowers are frozen (and possibly killed) in a late frost. Global warming notwithstanding, it’s not at all uncommon in the Upper Midwest where I’m from to see frost every month of the year. Across the Sun Belt, citrus growers routinely face frost as their crops ripen.

Most plants normally supercool to a few degrees below 32°F, but generally ice nucleators cover plant surfaces—leaves, stems, flowers—and initiate freezing. On clear, windless nights, heat loss into the open skies causes plants and other objects to become colder than the surrounding air. The air temperature may never dip below 32°F, but temperatures of leaves and the soil surface may fall below freezing and ice nucleators initiate freezing (radiation frost).

If moisture is present on a plant surface and there is an entry point (a wound, a broken epidermal hair, or a stomate), ice can form and propagate within the plant’s intercellular spaces. Ice crystals within cells are always lethal. But that’s not how damage is generally caused by frost, and the ability of a plant to adapt to seasonal cold plays no part. Instead, ice in the intercellular spaces causes water to flow out of the neighboring living cells into the intercellular spaces where it too freezes. As the amount of intercellular ice increases, more and more water flows out of cells. Ultimately, dehydration rather than freezing per se injures or kills the plant.

Many woody plants are not much susceptible to this sort of frost damage. Yews and oaks are examples. Research indicates that morphological features such as thick, waxy cuticles act as barriers to ice nucleation and propagation in these plants. In some plants, the propagation of intercellular ice is blocked from entering tender lateral shoots or blossoms. There has been some success on an experimental basis spraying hydrophobic particle films on the surface of tomato plants, which are tender and can be killed by frost, to block ice nucleation. But the current procedure is probably not worthwhile on a commercial basis and impractical for a home garden.

An obvious practical approach a gardener can take is to be sure that plants are well watered before a period of expected frost. Having fully turgid, nonstressed plants may prevent killing cellular dehydration that can accompany a growing-season frost. In the cranberry bogs of New England and Wisconsin, when there is threat of frost, commercial growers continuously apply water to cranberry vines with sprinklers. The rationale behind this is that as water freezes, heat is released. This is what is known as the “latent heat of fusion”, and it is enough to keep the vines from freezing. Citrus growers avoid allowing cool air to pool by keeping air moving with giant fans.

As days grow shorter and nights colder, annual herbaceous plants senesce and die, but perennial plants that are adapted to the temperate and boreal zones enter a period of dormancy and begin to acclimate to the cooler and ultimately freezing temperatures that a month or so earlier might have killed them. Despite 100 years of study, our understanding of how plants perceive low temperatures and respond by regulating gene expression and metabolism is incomplete. This is not too surprising really since cold adaptation is an exceeding complex trait that is controlled by a myriad of genes that in turn are influenced by a myriad of factors, and it is nearly impossible to model adequately.

The concept of “cold adaptation” is implicitly presented as if it were a unique, one-time event. This of course is a grand oversimplification. The climate in winter is no more stable than it is in summer. Winter begins on a particular calendar date, but cold temperatures do not usually conform. There are periods of intense cold, followed by warming trends, followed by intense cold. Plants experience these events and respond to them. In midwinter, a warming trend may induce a plant to partially deacclimate, but the next week the same plant may be subjected to intense cold. To survive, it must reacclimate.

In a purely descriptive simplistic sense, as a woody plant adapts to cold, water is flushed out of the cells to the intercellular spaces, where it freezes, and water further flows out in response to the intercellular ice. The composition of fats and proteins in the increasingly permeable cell membrane changes, and salts, sugars, and proteins are synthesized that are concentrated in the living cells and increase the solute concentration, acting as “antifreeze”. Freezing and killing dehydration do not occur.

The ability to maintain this state, where intercellular water is frozen but the adjacent cells remain viable and intracellular ice nucleation is suppressed at very low temperatures is called “deep supercooling”. What allows small quantities of water within cells to avoid freezing, despite the proximity of extracellular ice and low temperature, is poorly understood. The ability to supercool seems to be related to the cell wall structure and composition, but there are also adaptive features that must be under genetic control.

Over the last two decades, increasingly sophisticated molecular biology techniques have been developed for plants. More and more these tools have been applied to teasing apart the genes and their roles in cold acclimation of the weedy species Arabidopsis, the fruit fly of plants. Lots of progress has been made. But from an anthropomorphized view of evolution, the goals of Arabidopsis are quite different from those of woody temperate plants, and Arabidopsis has the capacity to survive only a few degrees of cooling below freezing. The knowledge gained from Arabidopsis will certainly aid in breeding more frost tolerant plants and crops, but understanding cold adaptation and deep supercooling may remain elusive.

This entry was posted on Tuesday, January 5th, 2010 at 12:49 pm and is filed under Guest Articles, Original Posts. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.
Follow Comments:
RSS Feed for This Post

32 Responses to “Frederick Dobbs On The Sub-Zero Garden”

  1. MDS said:

    As a Professor emeritus of meteorology (Rutgers) I am well aware of the concept (and fact) of supercooled water. Your description of a puddle of supercooled water in the street Is incredible. Perhaps others will contribute to this blog, and explain why.

    • Frederick Dobbs said:

      Dear MDS – Thanks for your comment. If you (or others) have any explanations, hypotheses, or guesses, I’d certainly welcome them. This has puzzled me since I observed it. The city uses lots of salt in winter, but that should only lower the freezing temperature by a degree or two, and the ambient temperature was well below where I would think salt would have any effect whatsoever.

      Any further thoughts?

  2. T. Brown said:

    Excellant article. Thanks for sharing.

    • Frederick Dobbs said:

      Dear T., Thanks for your kind words.

  3. Lee said:

    Thnak you for your newsletters. Not sure I am brite enough to follow all the scientific knowlege or philosophical newsletters, but they are one of my joys.

    • Frederick Dobbs said:

      Dear Lee, Glad you enjoy the blog; I always appreciate comments like yours.

  4. Karen said:

    This was a very informative article.
    Down here in Alabama, pansies are set out in October and they bloom all winter until the heat of July does them in. This past week we have experienced very cold temps for this area, Pansies are alive, but they appear to have just withdrawn into themselves.No longer blooming just siting there. Now I understand why. Thanks

    • Frederick Dobbs said:

      Dear Karen, Never having lived in the South, I didn’t know that you could grow pansies year round. I enjoyed reading your comments. Hope the pansies come back soon.

  5. Evelyn Baker said:

    I am a teacher, and student, of horticulture. I was so impressed with this piece. I would like to have permission to use it in my teaching.
    Thank you for writing such informative articles.
    E. Baker

    • Frederick Dobbs said:

      Dear Evelyn, Thanks very much; I’m flattered. Please credit Heronswood Nursery in the appropriate manner and it is fine with the company for you to use it.

  6. Kathleen Foulkes said:

    Very gutsy article,hopefully gardeners are still a breed of people who pay attention to and appreciate science (unlike a regrettably large portion of our nation’s population these days). Loved it!

    • Frederick Dobbs said:

      Hi Kathleen, Thank you for your comments. I share your sentiment.

  7. Jo LynnWilson said:

    WOW. Living in a moderate climate and frequently experiencing the ravages of frost, this information is incredibly interesting. I’m not so sure it will make any difference to my garden but it woke up my brain. Thanks for including this kind of stuff in your emails.

    • Frederick Dobbs said:

      Hi Jo Lynn, I am glad that the article interested you. I appreciate your message.

  8. Chad said:

    This jogged a memory of a similar situation i encountered and it turned out that the underground water system, which had a small leak, had kept the area immediatly aboveground warmer. It eventually froze, but burst later upon thaw. Possibly the area you experienced was warmer under surface for some reason too?. And when you happened by at the perfect moment to freeze the surface area?? Dunno but its all very interesting!! Thank you for all your insights on everything and keep ’em coming!…. Chad

    • Frederick Dobbs said:

      Thanks, Chad. That’s a good idea. My guess is that what caused the puddle to stay liquid was something anomalous as in your example. The rest of the winter I came back to that spot and it was never again unfrozen when it shouldn’t have been. So I’m still in the dark.

    • George said:

      My two cents is that there may be on very rare occasions a condition of water to respond to its surrounding atmosphere in a statistically freakish form. Since it is a substance that, as you point out, can be variable, it may be that this puddle was an anomaly of purity. Maybe its molecular structure was composed so that the usual rules of reaction to temperature did not apply. Someone told me once that ocean water is extremely variable. So perhaps it was something like this—a true oddity that you came across. Otherwise, I agree with Chad’s suggestion.

  9. GSS said:

    A wonderful writeup on a fascinating topic. The unfrozen puddle on your winder stroll is astonishing – I wonder how it could have been free of nucleate. Having worked in my early career on modeling the behavior of ice under high pressure and related crystalline structure changes – your writeup brought back memories of that work. Will look forward to more such discussions.

    • Frederick Dobbs said:

      Dear GSS, Thanks much for your comments. Ice is a fascinating subject, isn’t it?

  10. milt said:

    great article thanks

    • Frederick Dobbs said:

      Thank you, Milt.

  11. Cara said:

    Thanks! First time I ever understood why garlic mustard is so invasive. Now, is there any way of enriching the soil to encourage the growth of the mutualistic fungi?

    • Frederick Dobbs said:

      Dear Cara, There are commercial inoculants, but I don’t know how well (or if at all) they work. Overly cultivating your garden can disrupt mycorrhizal fungi, and some plants (cabbage-type plants) can suppress mycorrhizal fungi. Probably the simplest way to encourage mycorrhizal fungi in your garden is through mulches and composts. Mulches and composts improve soil and plant health and control weeds. They improve drainage, lower soil temperature in the summer, and insulate roots from cold in winter. They inhibit undesirable microorganisms such as soilborne pathogens and stimulate beneficial microorganisms, including mycorrhizal fungi.

  12. Pat said:

    A very good article. Just shows we don’t know everything yet and there is much work to do.

  13. Dennis said:

    I am suspicious of an alletropic effect from Tree of Heaven (Ailanthus altissima).
    In Lake Park (designed by Fredrick Law Olmsted)here in Milwaukee, a matched pair of Redbuds were planted on either side of the grand staircase about 10 years ago. The one on the south side of the staircase is half the size of the one on the north side. A tree of heaven had been removed prior to planting on the south side and may have contaminated the soil. All other growing conditions are identical.

    • Frederick Dobbs said:

      Dear Dennis, Good observation. I believe that the toxic compound(s) produced by tree of heaven do persist in the soil, so you may well be right. I found one reference that might be useful, if you feel like persuing it—Lawrence et al. (1991). The ecological impact of allelopathy in Ailanthus altissima (Simaroubaceae). Am. J. Bot. 78:948–958. It may be available on the internet, but if not and you’re in Madison, you can certainly get it at Steenbock Library.

      Lake Park is lovely this time of year.

  14. Claudine said:

    A very interesting article.
    I have a question that seems unrelated, but as the natives go. During my studies as a landscape designer, emphasis was on natives, which I use as often as possible. For me, though, some are invasive. Do we know for how long those plants have been growing on the US territory and could it be that they prevailed over more ancient plants?
    Seeds come from boats, logs, birds, wind, etc

    • Frederick Dobbs said:

      Great question, Claudine. I don’t know that anyone has stated how long a plant must have been growing in a region to be considered “native”. Is Kentucky blue grass native? It came to the USA with the colonists nearly 400 years ago. I’ve heard the argument made that a species is not really native if the germplasm didn’t actually come from the area where it is being cultivated; I’m talking about counties here. So, it may depend on whom you’re talking to.

      No doubt, as mountains rise and the climate changes (over eons), our current native plants have supplanted our former native plants. Quite rapid evolution (10,000 years say) is seen in Arctostaphylos spp. (manzanita). See

      I think you’re also quite right about native plants; they’re probably invasive as much as they’re opportunistic and maybe aggressive colonizer. Not all exotics are invasive. It’s only a small subset that show greater abundance, density, or competitive dominance in their introduced ranges—garlic mustard, purple loosestrife, and knapweed come to mind. But there are plenty of native plants that are opportunistic and aggressive colonizers. Common ragweed is an example and I believe it’s native to all 48 lower states—no one of course would think of using it horticulturally, but you get my meaning.

  15. pk said:

    I am amazed at your intelligence. Gotta love what you do…thanks for the lesson.

    • Frederick Dobbs said:

      Thanks for your kind words, pk.

  16. Sabra Ewing said:

    I would like to share that we have wondered if a red oak adjacent to a 30 x 30 mixed perennial island bed, is the culprit responsible for the partial stunting to perishing of all types of Yarrow. Of course this is the clients favorite flower! Yarrow does just fine in our other gardens in a 30 mile radius that have the same ph levels. Also, of all plants yarrow does just fine in the wild here in Vermont most nursery men and women respond ‘Yarrow?” but that’s easy to grow!”

    • Frederick Dobbs said:

      Dear Sabra, Thanks for your comment. I would agree that yarrow is pretty easy to grow generally, and it’s endemic pretty much everywhere in North America. I think allelopathy is probably not what’s going on. Had you not mentioned pH, I would have. Oak will of course happily grow a pH much lower than yarrow. Yarrow likes a sunny spot with good drainage and light soil. Could the problem be something as simple as this?

Follow Comments:
RSS Feed for This Post