very time the seasons change, we get a flood of gardening questions and requests for best practices. These questions are all over the proverbial horticultural map, ranging from topics like transplanting shrubs to growing native plants to tips on not killing flowers. Last week's (very) cold spell got us to thinking, so we asked our executive director for some pointers. In true Fred fashion, his counsel is highly educational and extraordinarily thorough. What follows is Out in the Cold - Part One, the first half of Fred's answers to pretty much any wintertime gardening-related question an Alabama gardener might have.
OUT IN THE COLD
by Fred Spicer, Executive Director, Birmingham Botanical Gardens
by Fred Spicer, Executive Director, Birmingham Botanical Gardens
It is specifically during frigid times like these that I am asked (mostly by non-gardeners) what Birmingham Botanical Gardens (BBG) is doing to protect our plants from the cold weather. My response, typically accompanied by a blank stare, is usually along the lines of, “What do you think we should be doing?” I’m not sure if people think we erect a temporary, heated dome over our 67 acres, or set up phalanxes of small heaters, or enshroud our living collections in big blankets – I really don’t know what they’re thinking. We’re not running a citrus plantation, after all. It seems to me that they’re hoping I will enlighten them with horticultural secrets carefully guarded by the gardening cognoscenti. Mostly, I think I disappoint them.
At this point, with winter firmly upon us, there’s really not much we can do. To be sure, we’re monitoring temperatures in our conservatory and greenhouses on an hourly basis (24/7), to make sure the heaters are working, but outside? Aside from some watering of new plantings and annuals (more on that later), not much at all. From our evaluative perspective, we are curious as to what our plants will do in the face of all weather, whether heat, drought or prolonged cold: what will they tell us? Our intention – following a collections mission of acquisition to determine suitable plants for our area – is to see what happens. But home gardeners don’t necessarily have the same mission, so I’ll discuss the factors that determine whether plants live, die, or partially die, in the winter, and a few things we’ve already done to maximize our potential success.
COLD HARDINESS IN PLANTS
Cold hardiness is determined by a complex set of factors including plant genetics, soil moisture, soil pH, microclimate, fertilization, absolute minimum temperatures, snow cover, and acclimatization. Such a topic is encyclopedic in its scope, but leaving genetic adaptability aside, much winter damage is caused by desiccation, or the drying out of plant tissues. Note here that ice – frozen water – is in a form that plants cannot use, making soil conditions essentially dry for the purpose of this discussion.
On the cellular level, cold damage to plant tissue has much to do with where and when water freezes. At the risk of oversimplification, all plant cells contain water, but water also exists in plants in intracellular spaces. If this water freezes before water in the cells freezes, it will provide insulation to the cells. This important adaptation is not found equally in all plants, but it does increase resistance to not only absolute low temperatures but also to long periods of sub-freezing temperatures. When water in cells freezes completely, cells will burst as the water expands, and the cells will die, leading to varying levels of necrosis (death) of leaf, bud and stem tissue. Such damage obviously varies from plant-to-plant, but can also vary among different parts of the same plant because hardiness varies from flower bud, to vegetative bud, to stems and to roots, from least to most hardy, respectively.
Remember that plants take up water through their roots and naturally lose water (at a more or less equal rate), as water vapor, primarily from openings (stomata) on their leaves (mostly the lower surface) or secondarily from stem surfaces. The degree of water movement, use, transfer and loss differs from the growing season (high) to the dormant season (low). Wind can increase water loss, as can relatively high heat, including, importantly, winter sun heating upper leaf surfaces (despite much cooler air temperatures). One plant adaptation that is especially useful in minimizing moisture loss is the minimization of leaf surface area seen most readily in conifers, cone-bearing plants that express their leaves in needles (pines), scales (junipers) and awls (Cryptomeria); this can also be seen in plants like cacti where leaves have evolved into spines.
Another adaptation is the curling or drooping of leaves of broad-leaf evergreens such as anise (Illicium spp.) during sub-freezing temperatures; leaf surface area is minimized and more stomata are closed. Note that many conifers, and certainly cacti, are also highly drought tolerant, and that broadleaf evergreen leaves curl and droop in drought, and you immediately see the broad benefits of such adaptations. Still another moisture-conserving adaptation is the complete loss of leaves seen in deciduous plants. In temperate climates like ours (with more or less well-defined warm and cold seasons) leaf loss accompanies dormancy during the coldest part of the year. In other climates such dormancy occurs during seasonal dry periods. The advantage of dropping some or all of your leaves to conserve moisture is readily apparent.
The more extensively soil is frozen, the less water that is unavailable for root uptake; combine that with ongoing moisture loss through large, fleshy leaves and the special cold-weather stress on broadleaf evergreens is easily understood. A simple fact of gardening is that as you move north into climates with colder winters, fewer broadleaf evergreens are hardy, and their use declines: move south and the opposite is true.
Plants also actively change the chemical composition of water seasonally, within and between their cells, essentially producing freeze-resistant compounds that provide additional protection against the cold. That process is tied to acclimatization, how plants get ready for the winter ahead, and de-acclimatization, how they prepare for spring. Fully acclimatized plants are at their maximum potential cold hardiness. For example, a crape myrtle (Lagerstroemia sp.) in active growth in late September is very susceptible to even a light frost, which will severely damage leaf and stem tissue. The same temperatures in late January would have no effect. On the other side of the calendar, Asian magnolias such as saucer magnolia (Magnolia × soulangiana cvs.) can be severely damaged by frosts that occur in late winter following periods of relative warmth. In both illustrations, there is a timing problem as the plants’ “depth” of dormancy does not match environmental conditions (temperatures), and injury results; the former has not fully entered dormancy and the latter has been coaxed out of dormancy.
Dormancy is complex and like cold hardiness, is tied to plant genetics. It is influenced by day length and changing temperatures. Some plants react more strongly to one or to the other, or (mostly) combinations of both. If plants are subjected to a gradual cool-down as days shorten through the fall, and up-and-down swings in daily temperature extremes are not dramatic, plants enter dormancy gradually and fully. Combine that with ample soil moisture and it adds up to conditions that are excellent for optimal potential hardiness. These conditions match the fall of 2009 in Birmingham, so all things being equal, plants should have been at their optimum potential hardiness for this early January Arctic blast.
In nature, plants germinate and grow where seeds fall and conditions are suitable for their growth, over time. Gardening, in one sense, involves matching plant preferences with locations in the garden with similar conditions. We’ve all heard the axiom “right plant, right place”, and as far as survivability goes, that a good start. Here’s where microclimate enters the discussion. Climate is weather over time; microclimate is the weather over time in a particular location, whether that location is broadly defined as Birmingham, Alabama; or specifically defined as the north side of my Cahaba Heights home, protected from cold winter winds from the northwest by a 6’ fence, out of the warming sun, under the dryer vent, next to the foundation, in well-prepared, well-drained, highly organic, sandy loam soil with normal nitrogen levels.
Compare Birmingham to Chicago and those microclimates are vastly different. Compare that fifteen-square-foot location at my home, to another one on the south side, in full blasting sun, in heavy clay soil at the bottom of a two-foot-deep rain garden beneath a downspout, and those microclimates are different as well. Obviously not as much as the first example, but still: there are many plants that will thrive in that first location that cannot be grown in the second, and vice-versa. Determining plant placement and manipulating microclimate are two of the gardener’s most essential practices. Cold hardiness (and overall survivability) is strongly governed by microclimate as it can affect temperature (especially warming on broadleaf evergreen leaves), soil moisture availability and air drainage (cold air moves downhill and collects in valleys), conditions that a cultivated plant cannot “choose” for itself.
Allow this small digression: if success is to be duplicated, observing well-grown plants must involve your gaining knowledge of microclimate, whether your observing occurs in garden (artificial) or “wild” (natural) settings. It bears mentioning that native plants all over the world are already extremely well-adapted to their local climate and, microclimate aside, are predisposed to handle it. Alabama native plants are generally not bothered by the up-and-down temperature swings we experience in the fall, winter and spring. Is damage possible from a prolonged cold snap like this one or an early or late frost? Sure. Is it likely? No.
CULTURAL PRACTICES AFFECTING HARDINESS
Certain gardening practices can affect cold hardiness. High soil nitrogen levels will act to keep plants in a more-or-less active state and will stymie the process of entering dormancy. If the plant in question is an annual that you’re trying to keep flowering until it (predictably) dies after the first frost: no big deal. Hardy plants, on the other hand, must be allowed to enter dormancy without hindrance. For those, lay off the high-nitrogen fertilization after 1 August. Apply fall fertilizers only after several hard frosts. Think: Thanksgiving.
Pruning always acts to stimulate new growth. If that growth emerges late in the growing season, it may not acclimatize (harden off) for winter and could be damaged by severe cold. Do not prune woody plants between 15 August and Thanksgiving, except for removing dead wood, or perhaps dead-heading plants like butterfly bush (Buddliea cvs.) that get cut back hard in the spring anyway. (Who cares if they die back a little bit?) Clearly, timing of pruning is important. For example, the late spring and early summer-flowering big-leaf hydrangeas (Hydrangea macrophylla cvs.) must be pruned – if pruning is desired at all – as soon as flowers begin to fade (turn brown). New growth, which contains the buds that will develop into next year’s flowers, will then be stimulated. This growth must fully elongate and harden off completely for maximum hardiness to be achieved. Untimely pruning is one of the main reasons these plants fail to flower on a regular basis.
Planting time can also affect hardiness. Small plants with small root masses can easily be heaved up and out of the soil by the freeze-thaw cycles experienced as temperatures dip below freezing at night and rise above freezing during the day. Exposed roots will dry out and damage and death can occur. Established plants, those with a larger root system, are better able to hold themselves in the soil. They also have increased ability to take up water. Both point to increasing potential hardiness. If you’re trying to “push the gardening envelope” by trying to grow plants whose hardiness is not known, or is known to be marginal for our area, you’re better off planting them in the spring and establishing them over a full growing season. Fall planting – while being the absolute best time for planting almost all hardy plants, especially woody plants – can work to decrease hardiness in somewhat tender plants.
The concept of hardiness zones provides a basic, but extremely useful, comparative tool for gardeners and a good beginning point. The cold hardiness map developed by the US National Arboretum (http://www.usna.usda.gov/Hardzone/hzm-se1.html) puts Birmingham in zone 7A with an average annual minimum temperature range of 5-10ºF (Huntsville is in 7B, Montgomery in 8A and Mobile in 8B). It is helpful to know that the map is based on data collected (only) from 1974-86. An updated map, broadly anticipated, is in the works with data from more collections sites and over a longer time-frame. It is expected that Birmingham will be moved into (the warmer) zone 8B, as it has been in the more-recently published map from the National Arbor Day Association, their first effort. (Note also that many plants are not adapted to our hot summers and, although fully winter hardy, will die instead from heat stress.)
According to National Weather Service records (http://www.srh.noaa.gov/bmx/?n=climo_winter2006bhm) collected in Birmingham since 1895, winter temperatures in Birmingham regularly dip below freezing and can remain there for a day or more. Here are the top three periods listed for continuous sub-freezing temps: 116 hours – 23-28 January 1940; 110 hours – 15-20 January 1977; 95 hours – 2-6 February 1996. On 13 February 1899, we sustained our record absolute low temperature of -10ºF; we reached -6ºF on 21 January 1985 (which followed -4ºF the previous day). I can imagine that each one of these cold weather events significantly re-set the horticultural clocks in our area, and damaged and killed many cultivated plants. January is historically our coldest month with an average daily temperature of 32ºF; December is next at 35ºF and February follows at 36ºF; March and November come in at 42ºF. Birmingham experienced its latest spring frost on 21 April 1986; I’m sure that was devastating as well. But these events are not the norm; our winters are generally short and relatively mild and the soil seldom freezes solid. When (if) it does, it’s normally not for long.
Stay tuned to Part II, coming soon!