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.
Shopping for a retaining wall: 1  Hiring an engineer to help design a retaining wall can run about $200-$500 or more, depending on the complexity and extent of the project. 2  Hiring an engineer is strongly recommended for walls over 3 feet high. Poured concrete costs about $30-$40 or more a square foot (usually with a minimum charge for small jobs), or about $1,500-$2,000 for a wall 30 inches high and 20 feet long.