ISA Soil and Water Management 2 — Questions and Answers

Question 1: What is the primary concern with overwatering established landscape trees?

It makes the leaves grow too large
It saturates the root zone, displacing oxygen and creating anaerobic conditions that lead to root suffocation and disease (Correct answer)
It causes excessive height growth
It dilutes soil nutrients beyond recovery

Correct answer: It saturates the root zone, displacing oxygen and creating anaerobic conditions that lead to root suffocation and disease

Overwatering fills soil pore spaces with water, displacing the oxygen that roots need for aerobic respiration. Extended saturated conditions lead to root death, root rot diseases (particularly Phytophthora), and progressive decline.

While water is essential for tree survival, excessive water is equally damaging. When all soil pores are filled with water (saturation), gas exchange ceases — oxygen cannot enter the soil and carbon dioxide cannot escape. Root cells deprived of oxygen switch to anaerobic respiration, which is inefficient and produces toxic byproducts like ethanol and acetaldehyde. Within hours to days, roots begin to die. Saturated conditions also favor Phytophthora and Pythium root rot pathogens that produce motile zoospores requiring free water to reach host roots. Symptoms of overwatering mimic drought stress (wilting, yellowing, leaf drop) because damaged roots cannot absorb water effectively — a paradox that often leads to even more watering. Proper irrigation should moisten the root zone thoroughly then allow partial drying before the next application.

Question 2: How does a perched water table form and why is it problematic for tree root health?

It forms when tree roots grow into a natural spring
It occurs at the interface between two soil layers of different texture, trapping water above the finer-textured layer and saturating roots in that zone (Correct answer)
It is caused by underground utility pipes blocking drainage
It only forms in coastal areas near sea level

Correct answer: It occurs at the interface between two soil layers of different texture, trapping water above the finer-textured layer and saturating roots in that zone

A perched water table forms when water moving downward through soil encounters an abrupt textural change (e.g., sand over clay), causing water to accumulate at the interface. This saturated zone suffocates roots growing at that depth.

A perched water table is a localized zone of saturation that forms above an impermeable or less permeable soil layer, independent of the regional water table. This commonly occurs in urban landscapes where soil profiles have been disturbed: fill soil placed over native clay, or sand-based planting soil installed in clay-lined beds. When water percolates through the upper layer and reaches the textural interface, capillary forces at the boundary resist the water's movement into the coarser or finer layer below, causing water to accumulate above the interface. Tree roots growing in this saturated zone experience the same oxygen deprivation as with overwatering. Arborists encountering trees with unexplained decline in apparently well-drained upper soil should investigate subsurface drainage by digging test holes or using a soil probe to detect saturated layers at depth.

Question 3: What is the recommended method for irrigating newly transplanted trees during their establishment period?

Frequent, light sprinklings every day
Deep, infrequent watering that moistens the entire root ball and surrounding soil, gradually reducing frequency as roots establish (Correct answer)
Only water during the first week after planting
Apply water only to the soil surface beyond the root ball

Correct answer: Deep, infrequent watering that moistens the entire root ball and surrounding soil, gradually reducing frequency as roots establish

Newly transplanted trees should receive deep watering that thoroughly moistens the root ball and surrounding backfill soil. Frequency should be higher initially (2-3 times per week) then gradually reduced to encourage root exploration into surrounding soil.

Transplant establishment is the critical period when the tree must regenerate roots into the surrounding soil to become self-sufficient. The root ball retains the original nursery soil, which may have different water-holding characteristics than the backfill and native soil. Watering must address both: the root ball (where existing roots are) and the interface zone (where new roots must grow). Deep, thorough watering encourages roots to grow downward and outward. Light, frequent watering keeps only the surface moist, promoting shallow root development. A general guideline is to water 2-3 times per week during the first growing season, reducing to once or twice per week in the second year, and allowing natural rainfall to support the tree by the third year — though this varies with climate, species, and soil type. Mulching the root zone (2-4 inches, away from the trunk) significantly reduces irrigation needs by conserving soil moisture.

Question 4: How does road salt (sodium chloride) damage trees growing along roadways?

It attracts deer that browse on branches
It increases soil sodium levels causing clay dispersion and poor drainage, draws water from roots through osmosis, and direct spray causes foliar desiccation (Correct answer)
It only affects trees through splash damage to bark
It creates acid rain when dissolved in water

Correct answer: It increases soil sodium levels causing clay dispersion and poor drainage, draws water from roots through osmosis, and direct spray causes foliar desiccation

Road salt damages trees through multiple pathways: soil sodium causes clay particles to disperse (destroying structure), salt in soil water draws water out of roots through osmosis, and salt spray on foliage causes desiccation and tissue death.

Road salt (NaCl) is one of the most pervasive stressors for roadside trees, causing damage through several mechanisms. In the soil: sodium ions displace calcium and magnesium on clay particles, causing clay dispersion and loss of soil structure; sodium accumulation raises soil osmotic potential, making it harder for roots to absorb water (physiological drought); chloride ions are taken up by roots and accumulate in leaf margins, causing marginal necrosis. Direct contact: salt spray from traffic splashes onto branches and evergreen foliage, drawing moisture from buds and needles through osmotic desiccation, causing browning and dieback on the road-facing side. Mitigation strategies include installing physical barriers, using salt-tolerant species in roadside plantings (honeylocust, red oak, baldcypress), applying gypsum (calcium sulfate) to soil to displace sodium, and ensuring adequate drainage to leach chlorides.

Question 5: What is the relationship between soil texture and plant-available water holding capacity?

Sandy soils hold the most plant-available water
Loam soils hold the most plant-available water because they balance drainage and retention better than sand or clay (Correct answer)
Clay soils hold the most plant-available water
Soil texture has no relationship to water availability

Correct answer: Loam soils hold the most plant-available water because they balance drainage and retention better than sand or clay

Loam soils (balanced mix of sand, silt, and clay) hold the most plant-available water. While clay soils hold more total water, much of it is held too tightly for roots to extract. Sandy soils drain too quickly to retain much available water.

Plant-available water (PAW) is the water held between field capacity (the upper limit after gravity drainage) and the permanent wilting point (the lower limit where plants can no longer extract water). While clay soils hold the most total water due to their enormous surface area and small pore sizes, much of that water is held by matric forces too strong for roots to overcome — it is 'unavailable' water. Sandy soils have large pores that drain readily, holding little water at any tension. Loam soils, with their mix of particle sizes, create a range of pore sizes that hold significant water at tensions roots can overcome. Silt loam soils typically have the highest PAW: approximately 2-2.5 inches of available water per foot of soil depth, compared to 0.5-1 inch for sand and 1.5-2 inches for clay. This is why loam soils are considered ideal for most landscape plants.

Question 6: Why is the concept of the 'critical root zone' important during construction near trees?

It defines where new trees can be planted near the construction
It defines the minimum area around a tree that must be protected from soil compaction, grade changes, and root severance to maintain tree health and stability (Correct answer)
It marks the area where tree removal is prohibited
It indicates where irrigation pipes should be installed

Correct answer: It defines the minimum area around a tree that must be protected from soil compaction, grade changes, and root severance to maintain tree health and stability

The critical root zone (CRZ) represents the minimum root area necessary for tree survival and structural stability. Damage within the CRZ from compaction, root cutting, or grade changes frequently leads to post-construction tree decline and potential failure.

The critical root zone (CRZ, also called the tree protection zone) is typically calculated as a radius of 1-1.5 feet per inch of trunk diameter at breast height (DBH). For a 20-inch tree, the CRZ would extend 20-30 feet from the trunk. This zone contains the concentration of absorptive and structural roots necessary for the tree's survival. Within the CRZ, construction activities must be restricted: no soil compaction from equipment or material storage, no grade changes (either filling or excavating), no root severance from trenching, and no chemical contamination. Root loss beyond 30-40% often proves fatal, and cutting roots on one side of the tree can cause structural instability and windthrow risk in that direction. Tree preservation during construction requires a written tree protection plan, physical fencing at the CRZ boundary, and monitoring during the project.

ISA Soil and Water Management 2

What is the primary concern with overwatering established landscape trees?