Durability

Different types of cedar have different strengths. White cedar resists cracking, while red cedar can become brittle and split while you are building the planter box. Cedar endures for decades outdoors or in, so the planter box can conceivably remain useful longer than the plants you grow in it. Except for pressure treated lumber, fir has reduced durability if the bottom of the planter box is directly on the ground. You may need to apply a wood preservative to the bottom of the planter box. If the plants in the box are for food, it is usually better to raise the planter box off the ground than to apply chemicals to the wood.

Strength and Stability

Unlike wood from Douglas fir and true firs, cedar wood naturally resists insects. Yellow cedar even resists termites, and is so strong that wood taken from dead trees on the ground is viable for many years. Cedar woods also defy moisture damage and decay, especially if the lumber is sawn from the heartwood. Cedar is easier to cut than Douglas fir and is easier to glue and nail. Use non-corroding nails and fasteners on cedar to avoid chemical reactions that cause black stains in the wood. Red cedar resists warping, curling and checking, but not as well as Douglas fir.

Painting and Staining

Both cedar and Douglas fir have generally straight grains that take well to paint and stains. Some blotching of paint may occur with lower grades of cedar. Top-grade vertical grain Douglas fir, which is often used to make high-end furniture, is smooth and usually free of knots. Vertical grain Douglas fir often has few, if any, knots, making it a good choice for a decorative indoor planter. Close-grained firs accept stain and paint well. Wood from true firs has a straight, fine grain that is easy to cut, sand, nail and glue. This wood can be finished with paint, lacquer, oil, paint or wax.

Weathering

White cedar ages to a silvery patina if the planter box is not stained or painted. Red cedar turns a dark, charcoal color. Douglas fir ages to a rich gray. True firs have lighter colored wood than cedars and Douglas firs, but are generally available for outdoor use as pressure treated wood, which may not be suitable for growing edible plants in the planter boxes.