Abstract
When managing American elderberry (Sambucus nigra L. ssp. canadensis (L.) R. Bolli [Caprifoliaceae]) and European elderberry (Sambucus nigra L. ssp. nigra) in northern latitudes, growers should consider species selection and placement to overcome unusual freezing events. In North Central Ohio, cultivars of American elderberry recovered more robustly from 2 consecutive polar vortices than did cultivars of European elderberry species. A trial planting of established Sambucus spp. were exposed to very low winter temperatures during freezing events of 2014 and 2015. Both species suffered severe winter dieback each year. American elderberry recovered by the second year following the freezing events. The winter-kill of European elderberry was cumulative over both freezing event years. European elderberry died back successively each year with total kill of younger plants and significant dieback of mature shrubs. American elderberry nominally recovered the spring following the first freeze but recovered completely by the second year after the freeze events. Because of the growth characteristics of European elderberry, there was little to no recovery from the trunk or base. Mature specimens regrew from root sprouts over a period of 3 y; original specimens of European elderberry never fully occurred.
Elderberry is a native understory and landscape plant providing browse for wildlife, cover, and feed for migratory birds, as well as a berry crop. Although most parts of the plant are toxic without proper preparation, elderberry products have been harvested for medicinal and culinary uses by many cultures. Flowers are dried for use in syrup, liqueurs, confectionary flavoring, and tea; berries are cooked for syrups, fermented into beverages, steeped in tinctures, and used to dye cloth. Because of its easily removed spongy pith, the stems have even been used to make whistles.
The USDA PLANTS database shows the American elderberry (Sambucus nigra L. ssp. canadensis (L.) R. Bolli [Caprifoliaceae]) native to most of the US and Canada, whereas the European elderberry (Sambucus nigra L. ssp. nigra) is shown to be introduced in several eastern states and Canada (USDA NRCS 2018). Other North American species include Sambucus ebulus L., dwarf elderberry introduced into New York and Canada; S. nigra L. ssp. cerulea (Raf.) R. Bolli, blue elderberry in western US and Canada; and S. racemosa L., red elderberry in most of the US and Canada but absent in the deep South. Each type of elderberry has distinct floral and fruit characteristics for the niche market buyer.
American and European elderberries are closely related and their taxonomy is confusing. Common names of red elderberry, black elderberry, or purple elderberry are found in the popular press, blog posts, and so forth, and the variation of names further exacerbates this problem. Technically, the American elderberry is Sambucus nigra L. subsp. canadensis and is synonymous to S. canadensis. European elderberry is Sambucus nigra L. subsp. nigra and is synonymous to S. nigra (USDA NRCS 2018).
European elderberry growth characteristics in cultivation are distinct from American elderberry. European elderberries tend to be single-trunked, sometimes multi-trunked, growing in a small tree form with more-or-less horizontal branching (Thomas and others 2009). American elderberries form clumps from vigorous underground rhizomes (Stang 1989). When young, some American elderberries will form a single-trunked shrub, but this growth habit progresses to the more typical clump form as the plant matures and will form a hedgerow over time.
Commercially in the US, American and European elderberries represent a niche crop with market demand dependent on popularity among a relatively small group of buyers—cooperatives, farm and roadside markets, hobby growers, and wild collecting satisfy market needs. Currently, the processors and the fresh market are pushing for continued development of new elderberry cultivars (Fin and others 2008; Thomas and others 2015). American and European elderberries are viable niche crops for northern Ohio fruit growers as well as for nursery production for the ornamental landscape.
Because of interest in new specialty markets, a trial planting of elderberries was funded by the USDA, Sustainable Agriculture Research and Education (SARE) in Nova, North Central Ohio, to study the local production possibilities as a niche crop (Malinich 2013). In 2010, 7 selections were planted in a block with at least 3 plants of each selection/variety. Both European elderberry and American elderberry were represented in the planting. Sambucus cerulea and S. racemosa were also planted but performed poorly and were subsequently removed from the trial. Additional selections of American and European elderberries were added to the block so a total of 15 cultivars were represented in the trial block (Table 1).
Sambucus species represented in the trial planting.
MAINTENANCE OF ELDERBERRY TRIAL BLOCK
All plants in the original trial are currently mature. The block is fertilized and limed according to soil test results. American elderberries are pruned by removing any canes more than 3 y old; hedgerows are thinned to approximately 1 cane per ft2 with one-third of the canes being 1st y, one-third 2nd y, and one-third 3rd y canes. European elderberries are pruned to thin the canopy and to encourage new growth. Both species require having any winter-killed or damaged branches removed each spring.
Weed growth is managed with a combination of herbicide, mulches (both composted wood chip and “mow-and-throw” grass/legume clippings), and hand pulling. The planting is trickle irrigated. Initially, no significant pest problems were experienced that required treatment; however, recent spotted wing Drosophila (SWD) problems require treatment during picking season.
POLAR VORTICES
During the winters of December 2013 to March 2014 and December 2014 to March 2015, extreme low temperatures settled over the region with overnight temperatures dropping below −23 °C (−10 °F). A regional recording weather station provided daily minimum temperatures for the area (OARDC 2017). The USDA plant hardiness zone lists the area as zone 6a with annual extreme minimum temperatures of −23 °C to −20 °C (−10 °F to −4 °F) (USDA ARS 2018). The winters spanning January through March of 2014 and January through March of 2015 were uncharacteristically cold for the area, dropping well below the historical norms. Polar vortex events, and unusually cold non-vortex temperatures, during both winters dropped the temperatures well below the annual extreme minimum with some nights dropping below −28 °C (−18.4 °F) for a sustained period of time (Figure 1).
Ten lowest temperatures during January, February, and March, 2013–2016. The black bar indicates the average annual extreme minimum temperature for the trial location, zone 6a (OARDC 2017).
COLD DAMAGE
Damage to crops and landscape plants in the region was apparent during summers of both years (author’s personal observations). Peach (Prunus persica (L.) Batsch [Rosaceae]) and Vitis vinifera L. grapes (Vitaceae) were killed in large numbers—farmers are still replacing plants lost due to the freeze events. Wholesale ornamental nurseries saw large losses. In landscapes, Acer palmatum Thunb. (Japanese maple [Aceraceae]) experienced trunk cracking, and many other zone 6a plants died unless otherwise protected.
In 2014, regular pruning of the elderberries was delayed until April because of field conditions (snow, temperature). A higher percentage of winter-killed branches was noted in all elderberry selections; however, differences in the response to the vortices by American and European elderberries was noted.
American Elderberry
During the first extreme cold event in 2014, all specimens of American elderberry had some canes killed to the ground. Of the American elderberry selections, 100% were moderately damaged, with dieback of the upper third of most canes and up to 25% cane loss to the ground. Following similar temperature events in 2015, dieback in both species was again evident. All American elderberry selections had distal cane dieback of less than one-third of the upper portion of the cane and most exhibited entire cane dieback in excess of normal for the trial planting. During summer 2015, American elderberry plants replaced their winter-killed canes. New first-year shoots were prolifically produced from underground rhizomes. Second- and third-year canes were pruned back to sound wood and fruited from lateral branches.
European Elderberry
In 2014, younger European elderberry plants were killed to the ground with very little regrowth during the season. Branches of mature European elderberry were killed entirely and some trunk damage was seen in the summer, evident as bark sloughing (Figure 2). Of European elderberry selections in the trial, 100% were significantly damaged after the initial freeze event with more than 50% branch dieback and some bark splitting and sloughing during the season. One- and 2-y-old European elderberry were killed outright after the first year.
Trunk dieback of European elderberry showing removal of dead wood on main trunks. Some regrowth is seen coming from the base 2 y after the freeze (2016).
After the 2015 freeze event, further dieback into the crowns of the European elderberries killed younger specimens that had survived the first freeze. Older European elderberries were further damaged with entire branches and some trunks killed. Snow cover and drifts were irregular through the planting so no correlation can be made to level of winter damage on the trunks with the depth of any snow, drift, or other protection. Regrowth by way of basal sprouting and root suckers was apparent on European elderberry after the second year of decline.
The oldest (tree-form) European elderberry specimens in the trial were 3 and 4 y old at the time of the event and began a slow recovery during the 2015 growing season. They sent up uncharacteristic new shoots from their base or nearby under their canopy (Figure 3). These new European elderberry canes grew rapidly while the remaining branches showed only moderate growth. During pruning, dead and dying trunks were reduced but not removed if they exhibited at least some growth. In subsequent years these European elderberry were removed or reduced to a few still-viable branches. Winter 2016 was comparatively mild with moderately low temperatures, well above the average extreme minimums. Dead trunks were removed from the European elderberry during 2015 and 2016 pruning. Very little winter damage was apparent on either species in 2016, although the weak European elderberry branches had continued to die back to below the severely damaged trunks.
European elderberry showing regrowth from the roots and trunk base the second and third years after the initial freeze (2016).
DIFFERENCES IN POST-FREEZE RECOVERY
The polar vortex and freezing events of 2014 and 2015 correlate to significant dieback of European elderberries in the trial planting. All selections of European elderberry were affected and some were killed outright after 2 contiguous y of below-normal temperatures. American elderberries, however, had some dieback but not to the extent of the European types. Also, the growth habit of the American elderberries was followed by replacement of the winter-killed canes the subsequent season. The growth habit of European elderberry did not allow for quick or easy replacement of the damaged trunks. Because the European varieties invest more in a permanent aboveground structure to produce fruiting wood, loss of that older wood necessitates a complete replacement of the trunk and (or) major side branches—a process requiring several years at least.
The continued decline of the European elderberry in 2015 may have been attributable to their loss of vigor from the prior year, not just the temperature. American elderberry had less dieback the second year. Loss of the European elderberry from freezing was directly related to the characteristic tree form of these plants. Without the ability to renew growth from roots or rhizomes on an annual basis, the European plants were forced to produce renewal growth uncharacteristic for those particular selections. Younger plants in the trial, less than 2 y old, did not recover from 2 successive y of abnormally low temperatures. They may have had an insufficient root system to generate the required new shoots and crowns.
Because both the European and the American elderberry were heavily damaged by 2 y of abnormally low winter temperatures, their sensitivity to temperature may be similar. But, the ability to produce new canes annually almost guarantees the successful recovery of the American elderberry (Figure 4). The inability of European elderberry to produce the same prolific growth makes its rejuvenation more tenuous.
Regrowth of American elderberry hedgerow 2 y after the initial freeze (2016).
CONCLUSION
Management of European or American elderberry requires a working knowledge of the growth habits of these 2 plants. A damaged block of American elderberry could easily be cared for by total rejuvenation, cutting the entire hedgerow down to the ground. European elderberry, by contrast, would require a few years to replace, even if the plant were well cared for—removal of the initial deadwood in year 1, followed by reduction of weak branches and selection of more vigorous replacements in year 2. Gradual reduction of the damaged parent plant over 2 y could enhance production of replacement plants from the roots.
These observations would indicate that growers could produce both crops but should be ready to lose canopy of European elderberries during extreme cold weather. Mature plants will apparently recover, but young plants of European elderberry would need winter protection in extreme years. Because of the mercurial nature of weather, it is not realistic to predict extreme freezing events with sufficient notice to apply additional protection (such as mulch or insulating row covers) to European elderberry trunks or scaffold branches.
Footnotes
Photos by Timothy J Malinich
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