"Landscape timber" can have more than one meaning. A landscape timber can be any log or lumber piece that's used in landscaping, as a decorative planter wall, as a border for a garden bed, as edging for a walkway or some other purpose.
It is often used, however, to refer to a specific type of lumber that is approximately 3 inches by 4 inches, with rounded edges; even building supply stores whose catalogs list timbers in a variety of styles and dimensions will probably produce this version if you just ask for "landscape timbers".
8 Feet Is Standard Length
Most landscape timbers are produced in 8-foot lengths, which is a standard construction dimension. Railroad ties salvaged from removed tracks are traditionally 9 feet, 6 inches; some building supply stores sell both used ties at that length and new ones not treated with creosote in 8-foot lengths.
Widths of landscape timbers can vary. The typical rounded-edge version is approximately 3 inches deep by 4 inches wide, but these measurements can vary slightly from timber to timber.
Some of these are produced as byproducts of plywood production, and are the inner section of a log after the outside layers have been peeled off for veneer sheets.
Most Are Squares or Rectangles
Most landscape timbers are square or rectangular. Many landscapers use 4-inch by 4-inch preservative-treated posts for landscaping; the flat sides stack well.
New untreated ties are 8 inches by 8 inches; salvaged ties approximate that dimension but usually range from 7 to 9 inches in width and depth. Other common sizes are 4 inches by 6 inches, 6 inches by 6 inches and 6 inches by 8 inches.
All of these timbers are 8 feet long.
Special Lengths Are Available
Timbers are available in special sizes. Some companies offer lengths up to 20 feet, although 8 feet is standard, and longer timbers may require special orders.
Some suppliers use specific dimensions, such as 5-1/2 inches by 5-1/2 inches or 5-1/2 inches by 7-1/2 inches, rather than the more common "dimensional" references of 6 inches by 6 inches or 6 inches by 8 inches. Dimensional lumber is planed smooth, so a 4-inch by 4-inch timber is actually 3-1/2 inches by 3-1/2 inches.