Special measures need to be taken when landscaping with trees in locations where excessive amounts of salt represent an environmental problem. High levels of salts in soil, in runoff or irrigation water, as well as airborne salt spray can impact the growth and health of trees and shrubs.
If any one of these conditions exist at a particular site, landscape planning should take into consideration the selection of plants which are known to have salt tolerance. With respect to salts in water and soil, the excessive quantities of salt are absorbed by the plant through water uptake, and have a negative impact on plant metabolism. Salts borne by ocean spray can extend inland some distance, wetting the leaves of plants; the water evaporates leaving behind a film of salts which interfere with plant respiration and the production of chlorophyll. Plants that can grow well in salty habitats are designed as halophytes, but they are relatively few in number and mostly grasses. In seaside locations where rainfall is regular, the salt spray accumulations are washed off the leaves; sensitive trees can be planted on the lee side of structures or windbreaks so they are protected from direct exposure to salty sea winds. Molecular biology studies of salt tolerance have revealed that it is a complex genetic trait, governed by multiple genes working in combination; this factor makes plant breeding for salt tolerance a huge challenge. Treeworld has gathered information on the reported salt tolerance of trees and shrubs in their inventory and listed it in the species description as low, medium or high, with some designated as medium to high. Information about this subject is less available than on drought tolerance, and in certain instances the salt tolerance of a plant is simply unknown. Around 70 trees in the Treeworld inventory are considered to be salt tolerant. Among these are the Buttonwoods (Conocarpus spp.); the Stoppers (Eugenia spp.); native Florida palms (Coccothrinax, Leucothrinax, Pseudophoenix, Serenoa); the Coccolobas and almost all native Florida trees which occur in hummocks. Another 70 or so Treeworld trees have moderate salt tolerance. Examples are Orchid Trees (Bauhinia spp.); Strong Bark (Bourreria succulenta); the Cassias (Cassia spp.); Loreto and Blackbead trees (Pithecellobium spp.) and species of Zanthoxylum.