Plants have two different types of 'transport' tissue. Xylem transports water and solutes from the roots to the leaves, phloem transports food from the leaves to the rest of the plant.
Wooden retaining walls start at around $20 per square foot including materials and installation if using cost-effective pressure treated lumber. Hardwoods such as redwood, Cyprus or mahogany will raise the price over $30 per surface square foot but add an element of beauty to your wall. Railroad ties are the most commonly used wood for a retaining wall, and prices for this type of wall cost between $25 and $30 per square foot of wall. Untreated or soft woods are not recommended for retaining wall construction due to their susceptibility to rot and water damage.
This can add a few thousand dollars to the job, depending on the length of the wall. Though experts generally recommend that retaining walls sit no higher than four feet without any additional support, it's possible to have a retaining wall of up to 10 feet high. These taller walls can cost nearly $20,000 while a six-foot-wall can cost around $13,000.