Yellow isn’t Good – At Least for Most Plants

By John C. Fech

As a general rule, most trees and shrubs are green. Yes, there are a few select species of desirable ornamentals with non-green leaves such as golden birch, golden euonymus, sunburst honeylocust, Gold Coast juniper and tri-color beech that are exceptions, as well as Norway maple and yellow buckeye that turn to a delightful yellow/orange/reddish hue in the fall, but typically, yellow color means “sick plant.”

It’s common to see both the exceptions and the customary in the sports field and campus landscape. The job before us as managers is to figure out why each appears the way they do and deal with it appropriately. For the plants with some sort of malady, following the 4 R’s is the best approach.

R = Review

First, consider the site. The natural tendency when a yellow plant is observed is to think that it needs an application of some sort of nutrient mix (a.k.a., fertilizer). However, trees and shrubs generally don’t need nearly as much fertilizer as turf, especially if they are growing near grass that is regularly fertilized. On average, trees and shrubs can perform quite well on about a third of the amount required for healthy turf growth. When adjacent to, or in the midst of, sports fields or maintained campus grounds, the small amount that percolates downward past the turf roots usually suffices. Thus, the first few review or investigation considerations should be influences other than a lack of nutrients.

In addition to a lack of certain nutrients, other factors that can cause leaves to turn yellow include drought, herbicide injury, soil compaction, various pathogens, high or low pH, poor root system establishment, deep planting and water-saturated soils. The use of non-tried-and-true species for the area can also result in a predominance of yellow leaves. If various maladies are commonly seen on a certain plant in two or three out of 10 years, then lack of winter hardiness, high humidity, appropriate elevation, fluctuating winter temperatures and the like commonly also lead to the development of yellow foliage. Determining which of these influences (or which combination) are causing the leaf symptoms is an important first step.

This type of diagnosis is not simple. It often requires years of experience and/or consultation with an ISA Certified Arborist or an experienced horticulturist. Training programs put on by ISA or various land grant universities are well worth the time and money to gain the investigative capacity to deal with confusing plant symptoms.

R= Repair

With the realization that many influences or causal agents can create yellow leaf symptoms, a four-step practical approach to repairing or correcting nutrient deficiency symptoms is reasonable.

1. Eliminate insect or disease pests as a possible cause.

If it is determined that an aphid, fungal pathogen or virus is responsible for the symptoms, then it’s wise to deal with that determined influence directly. If not, moving on to approach number 2 is the next course of action.

2. Improve root performance.

Roots are the unseen but essential functional structures that absorb nutrients and water and support the structural stability of the plant. First, examine and possibly enhance the mulch layer over the roots. It’s best to replicate Mother Nature, which commonly places two to three inches of an organic wood chip type (not rock or rubber) of plant residue over them. Second, monitor the soil moisture and maintain it as preferred by the plant. For most, that means moist, not soggy or dy. Third, investigate root arrangement with the use of an air spade. Once the upper soil layer has been excavated, tangled or decayed roots can be removed, which may result in increased vigor of the plant.

3. Consider application of a plant growth regulator such as paclobutrazol.

As often seen in sports turf applications, these products suppress the leaves and shoots temporarily, allowing the energy of the plant to be channeled into the roots. More roots usually produce a healthier plant.

4. Fertilization with macro and/or micronutrients.

Fertilization need is based on three factors – the symptoms presented by the needles or leaves, the relative growth rate of the plant in question to others of the same species, and soil testing. While nutrient requirements can be regional in nature, and thus the importance of consulting with local experts, the following is a good head start into the realm of woody plant nutrient supplementation:

Primary nutrient – Nitrogen.  While all elements are essential, nitrogen is considered a primary nutrient. The symptoms of deficiency are slightly different for evergreen/coniferous plants and broadleaf/deciduous plants.

• Deciduous – leaves are uniformly yellowish green, more pronounced in older leaves. Leaves tend to be small and thin, have deeper than average fall color and may drop early. The shoots are short and smaller in diameter than usual and may be reddish or reddish brown.
• Conifers – needles are yellowish, short and close together. Older plants exhibit poor needle retention and lower crowns may yellow, while upper crowns stay green.

Minor nutrient – Iron. Iron deficiency is a common symptom, especially in soils that are high in pH.

• Deciduous – young leaves are yellow with contrasting narrow green veins, often with the older basal leaves remaining darker green. Exposed leaves are bleached and eventually exhibit apical or marginal scorch. Leaves may be small, with these symptoms being more severe during cold, wet springs. The shoot length is usually normal, but the diameter is often thinner than normal. Twig tieback and defoliation usually occur when deficiency is moderate to severe.
• Conifers – new growth is short, stunted and chlorotic, while the older needles and lower crown remain green. In seedlings, the cotyledons commonly remain green.

Minor nutrient – Manganese. Manganese symptoms often resemble iron deficiency symptoms to some degree. Determining which is responsible can be difficult.

• Deciduous – new leaves are commonly yellow to whitish, with green tissue showing along the veins. Brown dead spots often develop between veins.
• Conifers – leaves are smaller and lighter green than normal, commonly with interveinal chlorosis. Shoots are often shorter than normal.

Soil testing

A soil test is a great step before applying any products. Even simple tests will provide a baseline for a variety of macro and micronutrients, as well as organic matter, pH, cation exchange capacity and soil type. While not the be-all, end-all of diagnosing a yellow leaf/needle plant, soil testing is a good tool in the toolbox. The following guidelines help provide more accurate results than a shotgun, random approach.

• When? Same month every year. Soil temperature, use patterns and nutrient release are often tied to nutrient availability.
• Where? Match the location of the roots to the sites of the testing.
• How many? Five samples 2-6 inches deep, plus five samples 6-18 inches deep.
• How? Mix together in a plastic (non-metal) bucket. Total ≅  2 cups soil. A common soil testing probe can be used for collecting samples. Exclude any turf, turf roots and woody plant roots from the soil to be sampled.
• Ask lab. Request recommendations in pounds /1,000 square feet.

Unfortunately, optimal levels for the various nutrients are not well documented. Dr. Carl Whitcomb and Dr. Linda Chalker-Scott have published some general guidelines, as have some local and regional soil labs. Fortunately, iron and manganese are required in relatively small quantities by most plants. The products Osmocote Plus and Micromax are good sources of micronutrients. Keep in mind that nitrogen tests are temporary in nature, as it is a mobile nutrient. As such, tests for nitrogen provide a “snapshot” value only, and often are unreliable from month to month.
Unfortunately, optimal levels for the various nutrients are not well documented. Dr. Carl Whitcomb and Dr. Linda Chalker-Scott have published some general guidelines, as have some local and regional soil labs. Fortunately, iron and manganese are required in relatively small quantities by most plants. The products Osmocote Plus and Micromax are good sources of micronutrients. Keep in mind that nitrogen tests are temporary in nature, as it is a mobile nutrient. As such, tests for nitrogen provide a “snapshot” value only, and often are unreliable from month to month.

High/low soil pH

Though nitrogen is hard to test for, at least it tends to be available across a wide range of soil pH, which helps with plant uptake. Most micronutrients, such as iron, magnesium and manganese on the other hand, are pH dependent, being readily available or unavailable. For example, at pH 7-9 magnesium is readily available, and much less so at more acidic pH ranges. Iron and manganese are just the opposite, being available at low pH values, in the 5-7 range, and unavailable or tied up at higher pH ranges.

Knowing this information is one thing; being able to do something about it is quite another. The bottom-line question is whether changing soil pH is a viable option for woody plants in the sports field or campus landscape. Two factors conflict with the capacity to change soil pH for trees and shrubs. First, the soil volume, especially for trees. For large woody plants, approximately 40% of the absorptive roots are near the base of the tree, another 40% under the canopy as it extends to the drip line, and the remaining 20% extending outward as far as the height of the tree. It’s simply a large rootzone volume.

The other consideration is the difficulty in lowering and raising soil pH for the nutrients that are pH dependent for availability. For example, lowering the pH from 8 to 6.5 would require 25-50 pounds of sulfur to be broadcast and worked into the upper 6 inches of soil under the tree canopy. Some relief may be achieved by simple core aeration and topdressing with sulfur or ammonium sulfate; however, the results from these methods are quite variable. Similar but higher amounts of lime are required to raise the soil pH from 6 to 6.5. Both endeavors are affected by the amount of clay in the soil, requiring greater amounts in heavier clay soils.

R = Renew

Where stems with yellow leaves are localized, with symptoms being exhibited on a single or just a few branches, renewal may be in order. With trees, starting with the removal of certain branches may be successful, especially if they are infested with insects such as scale or infected with cankers. In approximately 30-40% of the cases, this results in a complete turnaround and return to health of the tree, which amounts to a great return on investment and retention of the symptomatic specimen. Pruning in early to mid-spring is the best timing in most situations.

The same guideline holds true for deciduous shrubs; removal of problematic stems at the ground level sometimes removes the symptoms and keeps the shrub in the landscape. If more than a few stems are involved, removal of all stems at the ground level is recommended, even though it may seem drastic. If the root system is otherwise healthy and the shrub has been established for three or more years, this type of renewal will eliminate the yellow leaves, increase the air circulation through the canopy and encourage new stems to replace the old ones. Total stem removal is not recommended for evergreen shrubs such as holly, arborvitae or boxwood, as it would kill the plant in most cases. Selective stem removal is encouraged instead.

R = Replace

More or less as a last resort, if repair and renewal are not successful, it’s wise to cut your losses and move on to something else – either another species that is better adapted to the site or a non-plant option such as a bench, water feature or small sculpture. Consult with local horticulturists and ISA Certified Arborists regarding good options and a thorough consideration of the right plant in the right place. Proper planting procedures go a long way to preventing future issues of this type. This includes digging a hole three times wider than the root ball but no deeper; installing the root mass such that the top lateral root is placed at or slightly above grade; and taking time to spread out any circling roots, covering them with 2-3 inches of wood chips and keeping the roots moist, but not soggy or dry going forward.

John C. Fech is a horticulturist with the University of Nebraska-Lincoln and Certified Arborist with the International Society of Arboriculture. The author of two books and more than 400 popular and trade journal articles, he focuses his time on teaching effective landscape maintenance techniques, water conservation, diagnosing turf and ornamental problems, and encouraging effective bilingual communication in the green industry.