Trellis Size Calculator

Match the trellis to the plant, not the garden. A good rule is 6 to 8 feet above the soil for most climbers: pole beans climb 6 to 9 feet (West Virginia University Extension), grapes train to a 6-foot top wire (UMass Extension), and climbing roses need supports 6 to 8 feet tall. Worked example from the planner: 3 climbing roses need a trellis 8 ft tall and 12 ft wide, with 4 to 6-inch lattice openings.

Bury at least 24 inches, or one third of the above-ground height, whichever is deeper. A 6-foot freestanding trellis needs 4x4 posts buried 24 inches, so you buy an 8-foot post (6 ft above + 2 ft buried). For an 8-foot trellis the burial becomes 32 inches, which means a 12-foot post. Wall-mounted trellises skip posts entirely and use brackets every 2 feet.

Use 2x4 posts for trellises up to 5 feet tall, 4x4 posts for 5 to 7 feet, and 4x4 posts with diagonal bracing (or 6x6 posts) above 7 feet. Space posts no more than 8 feet apart along the run. A 16-foot-long, 6-foot-tall freestanding trellis therefore needs 3 posts of 4x4.

Tendril climbers like clematis and peas need small openings of 2 inches or less on thin supports no thicker than 3/4 inch, because their tendrils cannot wrap thick wood. Climbing roses are not self-clinging (RHS) and must be tied in, so give them wider 4 to 6-inch openings that make tying easy and keep airflow moving to prevent disease.

Hold the trellis at least 3 to 4 inches off the wall so air circulates behind the foliage and you can reach in for pruning; the planner uses 4 inches. Plant the climber about 12 inches away from the wall base so rain can still reach the roots, and lean the first stems toward the trellis at roughly 45 degrees.

A walk-through arch is most stable at 5 to 7 feet wide. Bending a standard 16-foot cattle panel to a 7-foot span gives a peak of about 5.7 feet; mount the panel legs on posts or T-posts 1 to 1.5 feet off the ground to reach 6.5 to 7 feet of comfortable clearance. Going narrower makes the arch tall and tippy, going wider makes it short and weak.

Dig a 12-inch-diameter hole to the burial depth. For a 24-inch burial that is about 1.57 cubic feet of concrete, or 5 fifty-pound bags per post (one bag is roughly 0.375 cubic feet). Three posts on a 16-foot trellis therefore need 15 bags. Deeper 32-inch burials need about 6 bags per post.

Add 45-degree diagonal 2x4 braces every 8 feet on any freestanding trellis 6 feet or taller. On open, windy sites with trellises 7 feet or taller, also drive earth anchors with guy wires every 8 feet. Wall-mounted trellises over 8 feet tall should get mid-height anchors into the wall.

It depends on size and lumber prices, which you can edit in the cost estimator. A worked 8-by-8-foot freestanding trellis from the material planner needs 2 posts at $12 ($24), 2 rails at $6 ($12), 2 lattice panels at $45 ($90), 12 concrete bags at $6 ($72) and screws at $9: $207 total. Edit any price to match your local store.

Divide 96 inches by each plant's spacing: climbing roses at 4-foot spacing give 2 plants per panel, cucumbers at 12 inches give 8, and pole beans at 6 inches give 16. The size planner does this division for you once you pick the plant and enter the count.