Dieback on Broadleaf Evergreens
Broadleaf evergreens like rhododendrons, hollies, Leucothoe and mountain laurels took a beating this winter, in spite of what seemed to be an overall mild season. Many are exhibiting leaves have bronzed or turned completely brown. In the case of Rhododendrons, whole sections of plants appear wilted and dying. What has happened and how should these plants be treated? Should the affected parts be pruned out?
It’s likely that a combination of factors contributed to the damage being widley seen this spring, some of which is also being seen on conifers. While the winter overall was generally mild, and we had limited snow accumulations, the mild periods were punctuated with cold, and we experienced many windy days. Additionally, excessive moisture in the previous growing season may have contributed to root loss in poorly drained soils. In the case of many of the broadleaf evergreens such as hollies, mountain laurel and Leucothoe, damaged leaves are most likely due to the extremely windy conditions, coupled with variable temperatures. Unseasonably low temperatures in early March, when the sun was strengthening and days were growing longer, warmed leaf tissue, which then lost moisture through transpiration at a time when soils were frozen and the moisture could not be replenished.
Additionally, several days with winds of exceptional vigor (February 25th was notable for the particularly persistent wind with gusts between 40 and 60mph) contributing to moisture loss, again, when plants were unable to replenish. Looking even further back, November temperatures were colder than normal, while early January was unusually warm (58 degrees in Boston on January 1st). Snow cover, which serves as an insulator helping to moderate soil temperature, was notably absent in many regions, allowing for repeated freeze thaw cycles that can cause root injury in particularly shallow rooted plants like Kalmia, Rhododendron and Magnolia. Reports of the upper portions of plants showing browning and dead leaves, while lower and interior leaves remain unscathed, suggests that the injury on these plants is due to wind and cold temperatures and that those portions unaffected were protected by the limited snow cover or shielded by the remaining portions of the plant. In these cases, it’s quite possible that the damaged foliage will be lost but the wood and buds are still viable. Before pruning, check to see if stems are alive by scratching the bark with your thumbnail to see if the underlying wood is still green – if so, wait until late May or early June before pruning so that you can assess which parts remain alive. At that point, if portions fail to leaf out, prune back until you find living wood. Make a sloped pruning cut, ¼” above and sloping way from an outward facing shoot or bud.
If the wood and inner bark on damaged shoots are brown, examine the length of the stem to see if you find any breakage, signs of borers, or sunken, dark areas in the bark that might indicate of a pathogen like Botryosphaeria. The many species of Botryosphaeria affect a wide array of plants, including Rhododendron, Ilex, Acer, Malus, Prunus and Cornus. Damage can be seen throughout the year as foliage wilts and turns brown, or when branches fail to break dormancy in spring. The pathogen usually enters the vascular system through an injury or opening created by pruning, breakage, insect or deer damage, or other physical damage caused by weather or pests. It can remain in the plant, asymptomatic, until the plant is weakened by another stress. Signs of Botryosphaeria include sunken, dark-brown cankers along, and sometimes girdling, stems. The diseased wood is stained reddish-brown and, when cut in cross-section, may display a wedged-shaped darkened area. Tiny fungal fruiting structures dot the bark and spores are dispersed during wet periods, either by rain or overhead irrigation. Spores can also be transported by insects and pruning tools. Prune well below the damage, to an outward facing shoot or bud, or to the ground, and destroy any infected leaves and branches. Good fall or early winter cleanup around infected plants helps reduce subsequent infections, as spores overwinter in infected leaves and branches.
Rhododendron borer (Synanthedon rhododendri) damage often manifests itself during the growing season, but impacted shoots further stressed by winter conditions may show the effect this time of year. Look for the entry holes near the ground or in crotches of branches along the stem. When the borer is active in mid-late summer and again in early spring, frass and/or sawdust appear from the hole and may accumulate on the ground. Entrance and exit holes can be seen along the branches and can be pruned out and destroyed when larvae are in residence. The larvae pupate for two weeks in May with adult wasp-like clearwing moths emerging from late-May into early June. After mating, females restart the cycle by laying eggs near former tunnels, near pruning cuts, or in branch crotches. When the pupae emerge, they tunnel into the stem and begin feeding, and will overwinter in stems.
The sporadic dead branches visible throughout large and established rhododendrons may also be caused by root and crown rot from Phytophthora infection. Phytophthora is an organism that persists in poorly-drained, water-logged soils and proliferates when temperatures and moisture is high – just the conditions we experienced last summer and into the fall. It often gains a foothold when plants are planted too deeply, or in soils that are overly wet or compacted. Some plants experience stunted growth as Phytophthora spreads, followed by sudden branch die-back. Small areas of damage can be pruned out and, if excessive moisture is alleviated, the plant may survive. Heavily damaged plants are best removed as fungicides provide little relief from this organism. This disease persists in the soil, entering the root system and spreading upward, destroying the vascular system as it travels. Susceptible species, including Acer, Fagus, Ilex, Juniperus, Malus, Pieris, Prunus and Taxus should not be replanted where Phytophthora issues have been encountered. The damage to well-established plants that was seen this past winter may be the result of changing conditions – either locally around individual plants, or the broader changes to climate which have resulted in longer, wetter periods, extreme storms, and warmer than usual temperatures. These temperature and moisture shifts, combined with other environmental stresses, make it more important than ever to choose plants that are able to withstand a broad range of conditions. Those that are most sensitive to extremes are likely to be less successful in modern landscapes.
Joann Vieira, Director of Horticulture, The Trustees of Reservations