I am planning on constructing a 2-tier outdoor raised vegetable garden. I'd like to avoid any "gotchas". I live in the US Pacific Northwest, so three non-summer seasons a year are moist. My plan is:
My question: Are there any obvious "gotchas" with this plan?
First a question. What is going to be underneath your raised beds? Garden soil, lawn, patio, other?
If you're going to get into raised bed vegetable gardening I strongly suggest you look into Square Foot Gardening. It makes a lot of things easier when it comes to raised bed gardening and can also save you a considerable amount of money. For one thing, you learn that raised beds don't need to be that deep as long as you use a good potting soil. The author, Mel Bartholemew, recomends using 1x6 or 2x6 boards for the sides of your beds. That's only 5.5" thick. I built my square foot garden up to 7" because I wanted to add a lip around it. I like the way it looks but more importantly it helps me secure the plastic cover to extend the season or netting to keep birds out.
Some plants need deeper soil, potatoes for example, but you can always make accommodations for those or grow them in something like a grow bag.
You don't want to add gravel to the bottom. It doesn't help drainage, it actually reduces it. This is old advice that continues to get spread even though it's been proven wrong. When two mediums with different porosities meet, water does not travel well between them. When you water the potting soil from the top it will drain down to the gravel layer and then linger just above the gravel in the soil. In the PNW this can be a problem.
Railroad ties use creosote which is some pretty nasty stuff you probably don't want leaching into things you plan on eating. Same goes for older types of pressure treated lumber. The newer types of pressure treated lumber are supposedly better but I decided to play it safe and just use cedar. There are other cheaper choices that do pretty well such as southern yellow pine and douglas fir. All wood will eventually rot when in contact with soil and water. The amount of money you spend reduces the time between rebuilding your beds. If you don't mind rebuilding your beds every two years or so cheap framing lumber works. If you follow the SFG advice and keep your beds short it's not that big a deal to replace the boards.
I didn't use any anchors. I have 2 lengths of rebar in the back of the bed but they're primarily used to hold a trellis. The soil will keep the box from going anywhere and the sides of my boxes are screwed together.
You don't need a bottom for the raised beds. Some people like to put landscaping fabric if they're placing their beds over soil that previously had something planted to keep whatever seeds/roots may be in the soil from growing up through the bed. Same goes with tiers if you're doing what I think you're doing. The top tier will be open to the bottom tier making it extra deep.
Your plan is a little vague so that's about all the generic gotchas I can think of.
