5.3 Water Movement, Drainage, and Irrigation Decisions

Water Must Be Available, Not Excessive

Roots need water for uptake, transport, photosynthesis support, and cooling through transpiration. They also need oxygen for respiration. Soil water management is therefore not a simple instruction to keep soil wet. The arborist must ask whether water enters the soil, is stored in plant-available pores, drains before oxygen is excluded, and is replaced when tree demand exceeds supply.

A site can be too dry and too wet in the same season. Compacted soil may shed rain from the surface, then hold water in low spots where pores are small and oxygen is limited. A planting pit in heavy clay can act like a bowl if the surrounding soil drains slowly. A tree in a small paved opening can dry quickly during heat because roots have little volume to explore.

Soil Water Decision Table

Infiltration is the entry of water into the soil. Percolation is movement through the profile. Drainage is the removal of excess water from the rooting zone. Available water is the fraction roots can use, not the total amount present. Water held too tightly in very small pores may be unavailable, while water in large pores may drain quickly.

Irrigation decisions should be based on soil moisture and tree need. Newly planted trees usually depend on the root ball and nearby backfill until roots extend. Established trees may need deeper, less frequent watering during drought, but only if water can infiltrate. Sprinklers designed for turf can wet the surface while leaving deeper tree roots dry, or they can keep the trunk and mulch too wet.

Mulch helps water management by moderating temperature, reducing evaporation, and protecting structure from raindrop impact and traffic. It is not a substitute for drainage. A thick mulch mound against the trunk can hold moisture against bark and hide root collar problems. The correct practice is a broad, shallow mulch area kept away from direct trunk contact.

Drainage correction can be simple or complex. Sometimes the answer is to redirect irrigation heads, remove compacted surface layers, correct a berm, or choose a more tolerant species. Other sites require engineered drainage, soil replacement, suspended pavement systems, or changes to grade. Arborists should recognize when a problem exceeds a quick maintenance fix.

Water budgets include both supply and demand. Demand rises with heat, wind, sun exposure, leaf area, reflected pavement heat, and establishment stress. Supply depends on rainfall, irrigation amount, soil storage, rooting depth, and competition from turf or other plants. The exam may describe a tree near pavement and ask why normal rainfall is not enough. The answer may be restricted rooting volume and high evaporative demand, not a mysterious disease.

A strong recommendation tells the client how to monitor. Soil moisture can be checked by probe, hand feel, sensor, or inspection at an appropriate depth. Watering should change with weather and season. That practical adjustment is more defensible than a fixed calendar schedule.