The science of shrub care

By Brandon M. Gallagher Watson

Shrubs are key to landscapes in just about every setting. With all their various shapes, sizes, textures, and colors, shrubs dominate homeowner associations, commercial properties, corporate campuses, even residential areas <dash> and are often vital to the structure of the landscape design. While trees have arborists who specialize in their health, and turf care professionals can address grass issues, the care of shrubs almost always falls to the landscape managers. If your clients are looking to you to not only keep the shrubs trimmed and manicured, but healthy and vibrant as well, there are some things to be on the look out for.

Insects and diseases

Insect or disease issues on shrubs run a wide gamut from very species-specific issues (such as boxwood blight) to more general pests (such as aphids). Rather than get into any particular management strategy, it is useful to use broader categories such as “potentially fatal issues” and “cosmetic issues.” The first group is pretty straightforward; left unmanaged, this issue will likely lead to the death of the plant. This could be something such as the ficus whitefly (Singhiella simplex) or the aforementioned boxwood blight caused by the fungus Cylindrocladium buxicola. Left to their own devices, these pests will kill the host plant. Landscapes managers are left with the choice of either treat for the issue or plan to have the plant removed and replaced.

The second broad category of health concerns—the “cosmetic issues”—is a little less straightforward. Admittedly, I’m biased by my tree care background. In arboriculture, we are taught to educate the client on why aesthetic issues are “just unsightly—it won’t kill the tree so don’t worry too much about it.” Occasionally we run across the client who doesn’t want a leaf out of place, so we still offer some of these services, but it is not considered standard practice. Shrubs, however, are a little different than trees in this regard. We can talk a client into ignoring some leaf spots on their oak when the spots are 30 feet above their heads, but when the leaf spots are at eye-level, as they are with shrubs, the client may not be as easily swayed. Untreated cosmetic issues on shrubs lead to an undesirable plant that is simply removed and replaced. Thus, aesthetic issues really do become potentially fatal issues for shrubs, and warrant management more often than not.

Scouting for insect and disease issues should be a routine part of walking your clients’ properties. Symptoms to keep an eye out for include underdeveloped tissues (such as stunted leaves, shorted internodes, and failures of flowers or fruits), overdeveloped tissues (such as galls, witches brooms, or unusually profuse flowering), dead tissues (such as dieback, necrotic leaves, and wilting), and anything else that is an alteration of the plant’s normal appearance (such as usual color of leaves, spots on leaves, or uncharacteristic patterns on leaves). All of these diagnostic symptoms require at least a passing knowledge of the common shrubs and their cultivars in your area, of course. Shrubs are commonly propagated for unusual traits—such as colored leaves, stunted sizes, or even contorted growth—that do not occur commonly in nature. With a little experience it should be pretty straightforward to know a weeping shrub from a wilting shrub. If you are still unsure, there are many online resources available to assist with diagnostics.

Abiotic issues

Shrub health issues that are not directly the result of an invading insect or pathogen are often classified as “abiotic” issues, but, really, they could be called “site condition issues,” as poor growing conditions for the shrub are almost always the cause. Common issues include things such as lack of water causing drought stress, too much water caused by drainage problems, pollution/soil contamination, nutrient deficiencies, and herbicide damage. There can also be biotic issues that result from abiotic conditions. Examples of this would be things such as fungi causing leaf spots while the leaves keep getting wet from the sprinkler system. You can treat the leaf spots with a fungicide but the issue will continue to persist until the sprinkler system is adjusted.

Just like the insect and disease issues discussed previously, scouting for symptoms that may indicate an abiotic issue should be a regular part of walking a property. Contorted or stunted growth may be the result of herbicide damage. Off-color foliage, including interveinal chlorosis, can suggest a nutrient deficiency and possibly root system issues related to watering. Dead tissues can also indicate hydration irregularities. Depending on where in the country you are, dead tissues on leaves and twigs can also be symptoms of frost damage.

Growth control

While growth regulators have been around for many years, it has only been recently that they have become a standard operating practice for many landscape maintenance professionals. A big part of their rise in popularity has been the development of products that offer predictable and consistent growth control results. Plant growth regulators (PGRs) are utilized by property managers to reduce the amount that a shrub grows following a trimming event. The growth of treated shrubs is typically reduced 30 to 70 percent, and holds them for 8 to 12 weeks at a time (sometimes longer, depending on the species). This is beneficial to the property managers as PGRs allow you to focus labor toward the project areas most visible to the client rather than allocating them toward tasks such as maintaining a perimeter hedge or trimming parking lot shrubs.

Beyond the operation advantages PGRs provide, there are health benefits to the plant as well. First, just reducing the number of pruning events a shrub is subjected to has positives from a health standpoint. Pruning puts a plant into a growth “mindset,” meaning it is focusing its energy toward growth, often at the expense of allocating energy towards things such as root growth, defense compounds, or storage compounds. Second, modern plant growth regulators, such as paclobutrazol, help stimulate a plant into allocating growth away from vegetative growth and into these other categories such as root growth or defense compounds, making them more resistance to certain abiotic and biotic health threats. There is also research suggesting treated shrubs are more resistant to acute drought than untreated plants due to thicker leaves, increased root systems, and higher levels of absicic acid. This plant hormone is know as the “stress hormone,” and allows plants to quickly open and close their leaf stomata throughout the day in hot and dry conditions. Thus they can better balance the need to have them open to get carbon dioxide for photosynthesis with the detriment of having them open and losing water.

Application methods and timing

Whether we are talking about insects and disease, abiotic issues, or growth control, eventually the discussion needs to come around to applications. Shrubs have some unique challenges for application techniques that are not shared by other living things in the landscape, such as turf and trees. Applications of healthcare treatments in trees can be done a few different ways, including spray applications, trunk injections, and applications made to the soil at the base of the tree. Applications to turf can be made by spray application or by spreading granular products. Shrubs, however, are generally too small for trunk injection to be feasible and soil treatments have their challenges as well. Ideally, soil applications (both granular and liquid formulations) should be made to the mineral soil, but shrubs in the landscape have mulch, decorative stones, weed barrier fabrics or, more commonly, a combination of these products covering the soil beneath them. Not removing these barriers prior to application can result in uneven uptake and a further delay in efficacy of the treatments.

Timing of soil applications is also an issue in shrub care, as there is a lag between the treatment date and when you can expect to have efficacy of the product. Depending on the treatment being utilized, that lag can be anywhere from a few days to a matter of months. Combing this uncertainty with the difficulty of properly dosing treatments for shrubs, the vast majority of shrub healthcare treatments are performed by spray applications. Spray treatments eliminate the soil cover issues, they have fast efficacy, and they are fairly foolproof in terms of dosing accuracy. Spraying has fallen out of favor in arboriculture of late, as drift issues make scheduling and planning difficult. But spray remains the most popular management tool in shrub care with relatively short plants and significantly lower equipment costs than in tree care.

Unsung heroes

Shrubs are asked to do many things in the landscape: provide structure, provide color, provide texture, provide screening, and yet they remain overlooked by the casual viewer. When it comes to maintenance, the client often wants the shrubs to look perfect. Particularly in highly manicured landscape designs, shrubs are one of the top reasons a maintenance contractor may receive a callback. Keeping an eye out for potential health issues, and employing quick management tools can keep these unsung heroes looking their best, and keep your clients happy.


Brandon M. Gallagher Watson is a Certified Arborist and creative director at Rainbow Treecare Scientific Advancements.

This article originally appeared in SportsTurf sister publication of Landscape and Irrigation.