What's the best and/or easiest way to seal a planter box?
April 5, 2010 12:13 PM   Subscribe

What's the best and/or easiest way to seal a planter box? Extra points for vegetable-growing friendliness.

I built a planter box from non-pressure-treated lumber for my patio this weekend, but am kind of at a loss as to how to protect it from the moisture party that's going to be inside. I'd just stain/steal/forget about it, but I have a feeling vegetables aren't too fond of the chemicals that hide inside stain.

In case it matters, it's 20" deep, 8' long, 18" across.
posted by soma lkzx to Home & Garden (7 answers total) 2 users marked this as a favorite
Asphalt or tar is routinely used to seal the inside of terracotta pots. Ask for henry's at a hardware place.
posted by hortense at 12:22 PM on April 5, 2010

Last Spring, I sealed one with beeswax - it's about 2' x 4' x 18" deep, and I used a little more than a one-pound brick to seal just the interior. I would recommend using more than that, melting the wax on a double boiler, and painting it on with a (doomed) paint brush. Put extra in the corners.

Beekeepers (look for a honey supplier at your local farmer's market) seem to accumulate a lot of wax, and people only need so many fancy candles.

I sealed the outside of the box with polyurethane or similar.

It seems to have worked well! The difference in color between my box and my fiance's box (no interior lining/sealing) is dramatic. His is a dark gray, mine is a lightly weathered tan-gray. My corners are coming apart, though, mostly because I didn't do a good enough job connecting them.

Also, I have to be careful when digging in there not to disturb the wax.
posted by amtho at 12:29 PM on April 5, 2010 [1 favorite]

Another option would be to line the box with polythene and secure it with staples.

It's going to get wet anyway (from rain). Personally I'd just let the planter weather. It'll last a few years anyway and then you can knock together another one.
posted by le morte de bea arthur at 12:33 PM on April 5, 2010 [1 favorite]

Response by poster: Asphalt or tar is routinely used to seal the inside of terracotta pots. Ask for henry's at a hardware place.

I have a huge container of flashing cement left over from a roof leak, is that something that'd work?
posted by soma lkzx at 1:05 PM on April 5, 2010

I'm not sure this is worth all that much trouble. Untreated wood is likely to rot anyway, although you'll get a few years out of it. Maybe a few years is enough for this type of application. I would line the bottom with gravel, make sure there's good drainage, make sure it's up off the ground, and let it go.
posted by OmieWise at 1:13 PM on April 5, 2010

You can thin the flashing cement with mineral spirits slightly, so that it is brush-able and easy to apply.
posted by hortense at 2:23 PM on April 5, 2010

Some organic gardening sites recommend treating wood with a mix of parrafin and boiled linseed oil - here's a recipe. (we just built raised beds out of fir, which is somewhat naturally resistant, and haven't decided whether to do a coating on it or not).

I also found a fascinating description of homemade borax-based wood-preservative, which not only prevents rot but also pauses an ongoing rot process, as used on a log cabin, but I didn't read up on toxicity of borax for gardening.
posted by aimedwander at 7:41 AM on April 6, 2010

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