Thursday, January 21, 2010

Out in the Cold - Part Two

This is the second half of Out in the Cold, which contains more tips and factoids from our Executive Director, Fred Spicer, about wintertime gardening in Alabama. Read up, comment, and pass it along to fellow gardeners or "Curious Georges," if you will. If you have any questions not answered in parts one or two, give us a call at 205.414.3900 and we'll find someone who can answer your query!

Considering our climate, a major distinction to make initially is whether a plant is intended (or expected) to live out-of-doors through a Birmingham winter. For example, if you moved your fig (Ficus benjamina) outside for the summer – making a house plant a patio plant – and you haven’t brought it inside yet, it’s way too late. It’s dead. It’s not realistic to expect it to have lived. (The same could be said about the same plant having been planted in the ground; it’s not even close to being hardy.)

Normally winter-hardy plants, Japanese maple (Acer palmatum), for instance, grown outside in containers through the winter, have an excellent chance of living through a typical winter here. Moist, well-drained soil in the container is important. Given that, if the container can withstand the expansion of frozen soil without cracking, and if the plant is well-watered, hardy plants will have their best chance of survival. The hardier the plant, the longer it can withstand frozen soil and plunging temperatures. We don’t expect any damage to container plants at The Gardens from this recent weather. We’re watering as we can.

Recently-planted winter annuals like pansies (Viola spp.) should also be fine, although some might not grow much without a little more heat. Again, moist, well-drained soil is important. Freeze-thaw cycles can heave newly-planted annuals out of the ground, and the roots can get freeze-dried. Make sure plants are firm in the soil and provide supplemental irrigation as you can (and only if needed).  At The Gardens, we’re watering new annuals and we don’t expect any wholesale damage or loss.

Winter vegetables and herbs like lettuce (Lactuca spp.) and parsley (Petrosalinum crispum) vary in their ability to thrive through winter. Winter leaf vegetables can take some light frosts and many greens can take harder ones, but days on end of freezing weather will pretty much take them out, unless protected with some form of insulated cover (which must be promptly removed when above-freezing temperatures return). Very well-drained soil is important for herbs. Given that, woody herbs like sage (Salvia officinalis) and rosemary (Rosmarinus officinalis), and even thyme (Thymus spp.) should sail through days on end of sub-freezing temperatures with no injury. Herbaceous species like parsley, cilantro (Eryngium spp.) and chives (Allium schoenoprasum) will start suffering damage with heavy frosts, and they will go slowly downhill from there, unless covered, unless warmer temperatures return. At The Gardens, we expect to lose leaf crops like lettuce, and perhaps some greens and herbs as described above.

So-called tropical herbaceous perennials like canna (Canna spp.) and elephant ears (Colocasia spp.) have varying degrees of cold hardiness. Their hardiness is largely tied to freeze damage to roots so a warm microclimate is an investment in their survival. The longer air temperatures remain below freezing, the deeper the penetration of frost, and the more solid the soil will freeze. Heavy mulching can help keep roots insulated, but for susceptible plants it can exacerbate root rot if not removed promptly in advance of spring. At The Gardens, we have a number of these kinds of plants and anticipate some damage or death from winter cold (although not to cannas). We move indoors any we are serious about keeping and we always plant any questionably hardy plants in the late spring.

Despite nighttime lows of 14-16ºF, the unopened flower buds of wintersweet (Chimonanthus praecox) remain unscathed. This plant is in the Blount Plaza, in a wind-sheltered location with morning sun and filtered afternoon shade.

Open flowers on this hybrid mahonia (Mahonia × media ‘Arthur Menzies’) appear to have been killed by cold weather, but they were probably past before the frost. In certain plants, cold temperatures cause temporary cessation of flowering; note that the unopened buds are in perfect shape.

Winter-flowering cherries cannot withstand frost on their open flowers and in the Japanese Gardens, this Taiwan bell-flowered cherry (Prunus campanulatus) is no exception; unopened flowers may or may not be okay. If temperatures dip into the mid-single digits, branch dieback is likely.

The flowers of winter-flowering woody plants are more or less susceptible to frost and freeze damage. For us, we try to avoid planting these in the warmest possible locations, our thought being to keep their activity level moderate, rather than to heat them up as much as possible. Flowers of early deciduous cherries like Taiwan bell-flowered cherry (Prunus campanulata), autumn cherry (P. subhirtella ‘Autumnalis) and the hybrid Okame cherry (P. ×incam ‘Okame’) cannot withstand even light frosts. While the latter tends to open (and possibly lose) all its flowers at once, the former two open buds over several weeks and typically some flowers avoid frosts. Wintersweet (Chimonanthus praecox), winter daphne (Daphne odora), Chinese paper bush (Edgeworthia spp.), winter honeysuckle (Lonicera fragrantissima) and sweetbox (Sarcococca spp.) flowers can take light frosting with no damage; continuous freezing temperatures, particularly when the flowers are open (and at their most vulnerable), can eliminate them but such is the risk with these plants. Witchhazel (Hamamelis spp.) and (Pieris spp.) can easily handle repeated frosts, as can winter-flowering Mahonia species, and giant pussy willow (Salix chaenomeloides), both of which seem almost bullet-proof. Japanese camellia (Camellia japonica) flowers cannot handle hard frosts, nor can their partially open flower buds. Happily, they open sporadically over a month or more (following warm-weather triggers) and seldom lose all their flowers at one time unless we have a drastic absolute low temperature. At The Gardens, we anticipate these normal occurrences will happen again, as they typically do. We make a point to observe and enjoy them when they’re there, the better to deal with their possible demise.

Both absolute low temperatures and prolonged period of sub-freezing temperatures can damage flower buds of non-winter-flowering plants as well. Bigleaf hydrangea selections (Hydrangea macrophylla cvs.) vary greatly in the cold hardiness of their flower buds (and stems), which are produced the previous summer and fall, and are present on the plant through winter. Regardless of overall health, and despite proper pruning as mentioned above, winter may eliminate them. With the numerous cultivars available today, it is difficult to ascertain which will pull through unscathed in almost any given winter. Stick with the tried and true, experiment and learn from the results, and visit The Gardens this spring to see which ones bloom well. AldridgeBotanical Garden in Hoover also has a wonderful hydrangea collection.

Spring-flowering deciduous magnolias (Magnolia spp.), evergreen banana shrubs (formerly Michelia spp. and Mangletia spp., but now included in Magnolia), and countless other plants carry their flower buds through the winter. Winter damage can occur on these as well, and there’s little that can be done to avoid it. Peaches (Prunus persica) are notorious in their susceptibility to this damage, and commercial peach growers go to extreme lengths – heating entire orchards, using helicopters to generate air movement, spraying crops with water to create an insulating layer of ice – to minimize it. We do not practice this at The Gardens and we do not recommend this for homeowners.

Magnolias are especially prone to becoming active during mid-winter warm periods. As previously mentioned, a pronounced cold snap (even a light frost) can wreak havoc on these and cause massive loss of stem tissue along with the flower buds. Site these plants not in the warmest locations, but in cooler microclimates (shaded and with good air movement, on slopes, not in courtyards, not on south-facing walls) to minimize. Virtually every year, we experience freeze and frost damage to our early spring-flowering deciduous magnolias at The Gardens, and we anticipate such damage this spring. The lack of adaptability of these plants to our area is very frustrating, but it’s also educational. We are expanding our collection of the evergreen types, which seem to fare much better, possibly because they flower slightly later.

By far, we at The Gardens expect the most damage to occur on broadleaf evergreens because of the correlation between extended sub-freezing temperatures and foliar damage. Such damage is sometimes noticeable soon after the deep freeze has abated: leaves will develop brown patches, or turn brown overall and quickly abcise (fall off). Aside from optimizing planting location, watering well going into winter, maintaining adequate mulch (not too deep!) and minimizing fertilization, there’s not much that can be done. (Commercial anti-desiccants will help to reduce winter moisture loss, but are generally more useful in areas with pronounced, lengthy deep freezes.) Even with this kind of cold damage, we do not expect wholesale loss of established plants.

Evergreen herbaceous “perennials” and groundcovers are widely grown in our area and are important landscape plants. While they may be considered fully root hardy, repeated freezes act to damage leaves gradually, to more or less of a degree depending on the species and cultivar. Leaves on plants such as Lenten rose (Helleborus ×hybridus), evergreen Solomon’s seal (Disporopsis pernyi) and evergreen ferns like autumn fern (Dryopteris erythrosora), will suffer increasing damage to their leaves from cold weather as winter progresses and by March will probably look dreadful. Simply prune off the dead and dying leaves gradually or all at once (it’s easier if you do it before new growth ensues) and in short order new growth will replace them in spring. Note that this is an act of cultivation and is not necessary for the plant to continue to thrive.

Plants like Chinese sacred lily (Rohdea japonica), mondo grass (Ophiopogon spp.), and lilyturf (Liriope spp.) can also suffer cold weather damage to their leaves. Offensively damaged sacred lily leaves can be removed at any time, but a strong lawnmower works better (quicker) on the others if the leaves get trashed in the winter. Put it on the highest setting and finish this by early-March so as not to cut off emerging new growth.

Healthy and vigorous plants are better able to withstand all types of maladies, including winter damage; weak plants are always more susceptible. Relative to the cultural practices discussed earlier: except for annuals, at The Gardens we do not heavily fertilize our outdoor plants with nitrogen (at any time, really), nor do we prune in late summer-early fall. Plants are therefore able to enter fall with an activity state dictated by only the weather.

We strive to keep a 2-3” layer of mulch (leaf mould, pine straw or pine bark) on virtually all plantings to conserve moisture loss and provide a soil blanket to minimize rapid soil temperature swings. If conditions are very dry through the fall (and October is traditionally our driest month), we will selectively irrigate going into winter. This is especially true for new plantings (new that year) and winter annuals. We also practice winter watering when conditions are right, on a case-by-case basis.

Our mission at The Gardens involves the cultivation of many plants for which good hardiness information for our area is not known. We strive to plant these potentially tender-potentially hardy plants in the spring, and strive to provide the most propitious microclimate.

Patience is important when evaluating winter damage to plants. Such damage can be immediate and obvious, or can take some time to become manifest. So, too, the plants’ responses. Don’t be in a hurry to prune off cold-defoliated branches of broadleaf evergreens; wait until new growth ensues to determine if only the leaves were damaged, or if bud and stem tissue got hammered as well. For plants that suffered stem loss – even total loss of all above-ground parts – prune off the dead parts as noticed, but keep monitoring for signs of life emanating from the roots or crown. As mentioned, roots are much hardier than stems, and plants – most especially established plants – have an uncanny ability to re-sprout and re-grow, even after major above-ground damage. Dormant buds close to the ground or from the rootstock might be late to break in the spring compared to those of other, undamaged plants, but even after getting a late start are capable of making strong growth. For well-established plants, the speed and density of re-growth can be impressive.

Inexperienced gardeners often panic as media weather personalities fill the airwaves with evermore dire adjectives for cold. While they do an important public service warning us of icy roads, potential injury to outdoor pets, and the increased risk of damage to water pipes, people should not – under most winter conditions in Birmingham – worry about their plants. Gardeners should take whatever steps they can before winter sets in, and use periods of extremely cold weather to curl up by the fireplace with the season’s plant catalogs, and dream about the spring (and possibly, replacement plants)!

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