by Dr. Bert Cregg

ome of the most common questions I get Salt of the earth: when discussing conifers at various meetings baldcypress thrive or programs concern choosing conifers for on this heavily salted S difficult sites. Frequent questions include:What conifer parkway on the can I grow under shade? Is there a conifer that can outskirts of Chicago. tolerate flooding? Can conifers grow in alkaline soils? Conifers represent a large group of plants, but as a rule do best with good moisture, good drainage, slightly acidic soil pH, and part- to full-sun. As Susan Gruber, former coordinator of the MSU Nursery and Landscape Ag Tech Program, used (Photos courtesy of Sarah Tien, City Forester, Park Ridge, IL) City Forester, (Photos courtesy of Sarah Tien, to note in her conifer program, “Conifers do best on sites that are moist, yet well-drained. But then, who doesn’t?” In Michigan and much of the Midwest, the search for the perfect conifer site in the landscape is like the search for the Holy Grail. Virtually every site in this part of the world has at least one environmental factor that is suboptimal for tree growth. So what’s a nursery manager or landscaper to do? Fortunately there are conifers that break the mold and can tolerate adverse sites, including drought, poor drainage, alkaline soils, extreme shade, and exposure to road salt. Conifers have evolved in almost every habitat on earth in which plants can grow. Noted conifer expert Chub Harper points out, “When you’re looking for a conifer for a difficult spot, think about which trees naturally occur in those habitats. There’s almost always one species, and often more, that are adapted to the situation you’re dealing with.” With that thought in mind, here are some tough tree selections for tough places.

Dry sites Handling sites with excess drainage or low soil moisture-holding capacity is a specialty of many conifer species. Many pines, spruces, and junipers have evolved in parts of the world that are

40 The Michigan Landscape™ Shady characters. Hemlocks add elegance to shady landscapes. Hard pines (needles in fascicles of 2’s or 3’s) are often tough trees with good drought and salt tolerance. extremely arid. Generally speaking, as trees and also as evergreen shrubs In contrast to selecting a deciduous species in the hard pine group (pines and groundcovers. Another drought- conifer for wet sites, identifying a with needles in fascicles of twos or hardy groundcover is Russian cypress flood-tolerant evergreen conifer is threes) are very drought hardy. Among (Microbiota decussata). Conifer expert, more challenging. Arborvitae come pines that are native to Michigan, Chub Harper notes, “One of the the closest to fitting this bill. White this group includes jack pine (Pinus unique attributes of Microbiota is that it spruce (Picea glauca) can also withstand banksiana) and red pine (P. resinosa). is one of the few plants that grows well wet feet better than most other Other examples include mugo pine (P. under shade and drought.” conifers. In both cases, however, these mugo), ponderosa pine (P. ponderosa), trees are best described as tolerating Austrian pine (P. nigra), and Scots Wet sites intermittent flooding rather than weeks pine (P. sylvestris). Although trees in Although no one keeps statistics on of inundation. the latter group have some pest issues such things, it’s probably safe to say that in Michigan, their drought tolerance is poor drainage and wet site conditions High-pH soils truly remarkable. limit the use of conifers in Michigan Soil pH is probably next to poor As a Research Plant Physiologist landscapes more than any other drainage as a limiting soil factor for with the USDA Forest Service, I environmental condition. One usually conifers in Michigan. As a rule of conducted drought-tolerance research doesn’t have to look very far in mid- thumb, conifers grow best in soils on ponderosa pine and Scots pine. In Michigan to find conifers — typically between pH 5.5 and 6.5. The optimum greenhouse dry-down trials, seedlings pines or firs — that someone has planted range for individual species, however, of both species were able to survive in locations that are too wet. These trees can vary widely. Fraser fir Abies( fraseri), over two months without water. Based will struggle along for a few years with for example, grows in soils as low as on pressure chamber measurements, their crowns getting thinner and thinner pH 3.5 in its native North Carolina. seedlings of these species were able until they finally succumb. This pattern As soil pH increases, several mineral to withstand over 70 bars of plant is especially obvious in nurseries or elements — notably manganese and moisture stress, which is double Christmas tree plantations. The majority iron — become limiting and trees the amount of stress that most tree of the plantation is hale and hardy until may become chlorotic. For the most seedlings can tolerate. As previously you run across a depression or low spot part, firs are among the most sensitive noted, spruces and junipers are also very where the trees are declining. conifers to increased pH. In our exotic drought hardy. Colorado blue spruce Among landscape conifers, one of the fir research, however, corkbark fir and (Picea pungens), for example, is blue best choices for exceedingly wet sites sub-alpine firs have shown to be more because of the glaucous waxy coating is baldcypress (Taxodium distichum). tolerant of higher pH than other firs. on its needles. This coating acts to Following Chub Harper’s “look at Spruces, particularly white spruce and reduce needle moisture loss as well as where they occur naturally” rule, this is Serbian spruce (Picea omorika), usually protecting the needle’s photosynthetic an obvious choice. Baldcypress grow in make the list of alkaline-tolerant trees. apparatus from excessive UV radiation swamps and along rivers throughout the Several members of the Cupressaceae, at high elevation. Junipers, including southeastern United States, northward including Thuja and Juniperus, can also native Juniperus virginiana and into southern Illinois. This tree can tolerate alkaline soils. exotics such as Juniperus communis, withstand prolonged flooding and even are extremely water-use efficient plants saltwater intrusions. Other deciduous Shade because of their scale-like needles. conifers such as dawn redwood and Dealing with shade brings up Junipers can be useful on dry sites larch are also extremely flood tolerant. the important issue of acclimation

July/August 2005 41 versus adaptation. We often use these Salt exposure terms interchangeably, but there is Road salt presents a challenge for an important distinction. Adaptation selecting trees for sites near roadways refers to traits that a species evolves throughout Michigan and the upper over time, whereas acclimation refers Midwest. Road salt can damage trees to the response of an individual tree directly due to toxic aerial deposits of based on the environment in which it excess sodium and chloride. Accumulations is growing. An illustration here may be of road salt in soils can cause osmotic helpful. The hemlock genus (Tsuga) stress similar to “fertilizer burn,” and has evolved adaptive traits for low- high levels of soil sodium can reduce the light conditions (high photosynthetic uptake of essential elements such as efficiency, low-light compensation point potassium and magnesium. As with many for photosynthesis), and yet individual tough site issues, deciduous conifers, hemlock trees planted in partial or even particularly larches and baldcypress, are full sun can acclimate and grow well. frequently the trees of choice. Baldcypress Likewise, some trees that we regard consistently ranks as one of the most salt- as needing a lot of light, such as the tolerant trees available in the landscape. Norway spruce (Picea abies) or Eastern In the Chicago suburb of Park Ridge, Arizona corkbark fir is able to withstand high white pine (Pinus strobes), can acclimate Illinois, City Forester Sarah Tein reports soil pH better than most true firs. and grow well in shade. Where we often that baldcypress trees planted in a narrow encounter problems is when a tree is parkway along heavily salted roads are acclimated to one set of conditions thriving. Exposure to road salt presents (say, full sun in a nursery) and then an even greater challenge for evergreen transplanted to different conditions conifers since, unlike deciduous trees, (a shady landscape). If the shift is too their foliage is exposed to aerial salt drift dramatic or rapid, the tree may fail and salt splash throughout the winter. before having a chance to acclimate to As a group, pines vary widely in their its new environment. tolerance to road salt and include some So, given all that, who are the shady of the most salt-tolerant conifers (e.g., characters among conifers? Hemlocks, Austrian pine) and some of the most of course, usually start and end the sensitive (e.g., Eastern white pine). In conversation on shade-tolerant conifers. general, hard pines are more salt tolerant Their elegant form and texture adds than soft, although Japanese white pine an element of class to virtually any (Pinus parviflora) appears on several lists landscape. Yews (Taxus spp.), although of salt-tolerant trees. much disparaged due to indiscriminant pruning, are a great choice for shady Summary spots. In fact, shady areas are good places To successfully establish trees on difficult to plant yews and allow them to develop sites, nursery managers, landscapers, and their natural character without turning homeowners have two basic options. One them into hockey pucks or meatballs. is to modify the environment to reduce Members of the Cupressaceae are adapted plant stress. This option works best when to a variety of light conditions. Thuja the issue is drought or excess drainage, and Chamaecyparis can withstand shade, which can be addressed directly by though their growth is slower and their irrigation. Other site problems, however, form is more open in shade than in sun. such as alkaline soils or poor drainage, are Deciduous conifers such as dawn redwood, As mentioned earlier, Norway spruce difficult to remedy on a long-term basis. baldcypress and larches are great choices for and Eastern white pine can acclimate to sites with poor drainage. For example, adding elemental sulfur or shady conditions. For example, at Lake using ammonium-based fertilizer can lower Lansing Park near East Lansing, Norway soil pH, but the shift in pH is transient. spruce has naturalized and seedlings and Rather than struggling to make the site saplings are coming in under a dense right for the tree, a better and more overstory of mature spruces. sustainable option is to select trees that are adapted to the site.

42 The Michigan Landscape™