Boulevard Median Planting Failure under Salt and Poor Drainage
A new 1.1 km boulevard median is 3 m wide with 0.45 m of engineered soil over compacted subgrade. There is no permanent irrigation, only temporary establishment watering. The site receives winter deicing salt, summer reflected heat from asphalt, and dry prevailing wind. The contractor installed 180 bare-root shrubs of a species marketed as drought tolerant and 60 ornamental grasses. After one season, 35% of shrubs are dead, dieback is worse on the windward side, salt crust appears at the curb, soil pH is 8.2, electrical conductivity is 3.8 dS/m, and infiltration is 2 mm/h. The contractor blames the plants and proposes replacing them with the same palette. The client wants a 5-year warranty and low maintenance. The median drains only through two curb inlets that are 10 cm above the soil surface.
1. Diagnose the mortality and rank the causes. Explain why the contractor's drought-tolerant label is not enough.
Read the worked answer
Rank salt stress and poor drainage first, then compaction and low soil volume, high pH, and inadequate establishment water. EC 3.8 dS/m is saline for many shrubs; salt crust at the curb and windward dieback fit salt spray and soil accumulation. Infiltration at 2 mm/h means winter water and establishment water pond; inlets 10 cm above the soil make the median a shallow basin, so roots sit in saturated soil. Only 0.45 m of soil over compacted subgrade limits rooting and worsens summer drought. pH 8.2 can lock up iron and phosphorus. The drought-tolerant label addresses one stress only; it says nothing about salt, pH, wet feet, wind, or restricted soil volume. Replacement plants must meet all site stresses, not just low water use.
2. The client wants the same palette. What is your recommendation, and what soil and design changes must accompany any replanting?
Read the worked answer
Reject the same palette. Specify plants with documented tolerance to salt spray and soil EC, high pH, wind, and both wet winter and dry summer; use a mix of shrubs and grasses rather than one species. Soil: remove the fines-clogged layer, break up subgrade to at least 0.6 m where possible, add coarse organic matter, and provide positive drainage to inlets. Lower or reset the inlets so water does not stand 10 cm deep. Increase soil volume or use continuous planting trenches instead of small pits. Add curb cuts or slope the median to the inlets. If there is no permanent irrigation, select only species that establish with temporary water and specify a two-year watering protocol. Do not replant until drainage and salinity tests are acceptable; otherwise the new plants will fail the same way.
3. Define acceptance tests and warranty terms that protect the client.
Read the worked answer
At planting, test soil pH, EC, and infiltration; reject compacted, saline, or poorly draining soil. Inspect root flare, plant quality, mulch depth, and irrigation logs. During establishment, require survival counts at 12 and 24 months and no more than 10% mortality per species. For a five-year warranty, require 90% survival after two years and replacement of dead plants, with salt damage and drainage failure treated as warranty events. Final acceptance should include an infiltration test, as-built drainage, and no standing water 24 hours after a 25 mm rain. Exclude owner damage and vandalism, but do not let the contractor blame plants if the soil or drainage was defective. Hold retainage until the first full growing season is reviewed.