Building a greenhouse can be a rewarding accomplishment, but if it isn't done properly, the whole thing could blow away with one strong gust of wind. Although building a small, lightweight structure doesn't take much time, it does take preparation and hard work. Nothing is more frustrating than putting in the effort and time, only to watch the greenhouse cartwheel across the yard because it wasn't properly anchored. There are various methods to anchor a greenhouse, and the one you choose will depend on the size and construction of your greenhouse. A lightweight greenhouse is easier to anchor than a heavier one.
Hammer the 5-inch nails into the 4-by-4 posts at regular intervals. The nails will give the post "teeth" to keep it secure in the ground. Drive the nails in to a depth of 2 inches. Space the nails 4 to 6 inches apart. Use eight to 10 nails per post. Leave the top 6 inches of the post free of nails.
Dig a 1 1/2-foot-deep hole at each corner of the frame using a shovel or post hole digger. Insert the post into the hole. Leave 6 inches of the post above the ground.
Fill in around the post with dirt. Keep the post vertically straight. Tamp the dirt around the post to make sure it is secure. Attach the frame of your greenhouse to the outside edges of the posts.
Dig a 2-foot hole at each corner of the frame and one in the middle along the longest side of the frame. Tap the base of the hole with a 4-by-4 piece of lumber to level the bottom.
Build a 16-inch square wood frame using 2-by-8 boards and 1-inch nails. Insert the frame in the bottom of the hole. Fill the frame with mixed concrete.
Remove the nut and washer from the L-shaped anchor bolt and press into the concrete, keeping L-shape face down. Leave the threaded part of the anchor above the surface of the concrete.
Smooth the surface of the concrete using a concrete edging tool. Place the galvanised 4-inch post base over the top of the anchor bolt and press it lightly into the concrete. Remove the wooden frame while the concrete is still a little wet, but well-set.
Allow the concrete to harden for a day. Attach the nut and washer to secure the post base. Attach the 4-by-4 lumber to the post base with galvanised screws. Fill in the hole using dirt around the edges and gravel around the post to protect it from moisture. Build the greenhouse, using the secured 4-by-4 posts as anchors.
For a longer, lightweight greenhouse, use additional anchors along the length of the frame. Mix the concrete in a wheelbarrow with a shovel. You can add rebar in an "X" pattern to the concrete pad to make it stronger.
A lightweight greenhouse, longer than 14 feet will need an additional anchor along each side.