I was planning on building a bunch of shelves in my basement, and
bought a whole bunch of reclaimed 2x4's. Then my parents gave us a
heavy duty plastic shelf, which impressed me, and I decided to use those
down there instead.

This leaves me with a bunch of old 2x4's. They were very inexpensive
- about 90 cents each, and are in good shape.

One project that I've been wanting to do is make some square shaped
borders around a couple of fruit trees, and around the blueberry hedge I
just planted. I have the lumber, but my previous experiences of:

1) painting
2) staining
3) Thompson's Water Seal
4) polyurethaning

did not turn out that well. I am tempted to try using boiled linseed
oil on the wood, and see how well that works as a preservative. The
other times I've tried it in the past, the wood has always cracked and
warped within a couple of years. Considering how cheaply I can get
these, I'd be happy if they just lasted 6 years laying out there in the
yard.

Can any of you recommend a widely available method for treating these
boards so that they will last a bit better when used as borders for beds
and for mulch around fruit trees? I'd like to get on this project
within a few days, to make my yard look neater, so I thought I'd check
on here to see what the experienced folks had to say.

Thanks!