Leg lengths for an arbor or trellis may vary depending on the model or manufacturer. In all cases the length is sufficient to have at least 1' of the legs in the ground. First you dig the holes, insert the legs of the arbor/trellis and make it level. At this point the hole can be backfilled with dirt or filled with concrete to secure the arbor/trellis. In cases where a gate will be used on an arbor or strong winds may be encountered, it is recommended to use 4' long pressure treated lumber to assure the arbor/trellis will withstand the added pressure under these conditions. The procedure is to rip the wood to size so it will slip inside the legs at least 1' above ground with 3' of wood going into the ground. Concrete must be used to fill the hole for best results. The legs should be fastened to the wood using 1" screws. Using this technique will assure your free standing arbor or trellis can withstand the elements. All the arbors and trellises come with complete installation instructions. Good luck and enjoy your new arbor or trellis!