Thursday, January 26, 2012

The Raised Bed Club

My pal Harry called recently to tell me that he had located some rough cut, white oak boards to build raised garden beds. We discussed board dimensions, how to fasten them together, what to fill the new beds with. I knew that Harry was about to become a member of the Raised Bed Club. I have been a member for about 15 years and it is probably the single most productive gardening step I’ve ever taken. I’ll outline some of the benefits and details here. If you want to take your vegetable gardening to another level, consider raised beds.

So, what’s the big deal with raised beds? For many of us (and certainly my original motivation) it is to gain the ability to garden in poorly drained soils… or to get access to garden soil earlier in the season. Raising the soil profile even a few inches creates gravitational pull on water that leaves it better drained. Many folks think that it is the media in the bed that is creating all of that drainage, and to certain extent, it does play a role. But even raising the existing soil on a site improves drainage dramatically. And if you are worrying about the task of importing a lot of stuff to fill you new raised bed… stop. You can accomplish a lot by simply using the existing soil.

Now, to talk out of both sides of my mouth, as they say…. let’s not ignore the potential to really improve soil texture and structure by amending that stuff we’ll fill those beds with. If you can add good organic matter to your existing garden soil in a raised bed you will see additional benefits. Where do you go for good organic matter? It’s everywhere. If you live in Southeastern Pennsylvania you are within an hour’s drive of some of the best compost money can buy. And sometimes it’s free! Spent mushroom compost is a beautiful thing. Many garden centers sell it. As you get closer to Kennet Square in Chester County it becomes more plentiful and cheaper. Since this stuff is often a waste product of the mushroom industry, there are mountains of it. It is simply the growing medium that several crops of mushrooms have been living in. It is high in organic matter and nutrients. Mix about 25 % mushroom compost, by volume, into even the crummiest soil and you have a decent growing media. Sure, do a soil test after you make this mix and adjust as necessary but that mushroom soil is nutrient rich and near neutral pH.

Composted leaves are stockpiled in many communities. While these will be less nutrient rich than mushroom compost, they do provide wonderful soil textural qualities. Again, adding about 25 %, by volume, is a good start.

Manure is magnificent. Find one of those old barnyard piles that has been ageing like a fine wine and you have hit the jackpot.

OK you get the idea. Find some organic matter. Amend your existing soil by incorporating about 25 % of what you can find with the existing soil. Then soil test and adjust fertility as needed. This will get you started. As time goes by you will fine tune that media in the beds. It will settle and shrink over time so yearly additions are needed.

Some garden centers and mulch suppliers sell a garden soil mix in bulk, by the cubic yard. Most will deliver. Tell them your bed dimensions and they will tell you how much you need. A bed that measures 8 feet by 3 feet by 9 inches will require 18 cubic feet… less than a cubic yard (27 square feet).

Rock raised bed
Wooden boards are probably material most people use to construct beds but many things can do the job. Rocks work. So do concrete blocks. I suppose you could use the fancy fake rock wall materials that are so popular. Metal, plastic, you name it. In fact, a raised bed can be borderless if you’d like. But the tidiness of wooden raised beds is nice. What kind of wood? Since there will definitely be contact with soil, a rot resistant wood is important if you want the beds to last more than a couple of years. Pressure treated wood sold these days does not contain arsenic.

Borderless Bed
Copper is used instead to resist the rots. Got old pressure treated wood with arsenic? There is pretty good evidence that this does not pose a great threat, but if you are going to spend one moment worrying about it (or trying to convince someone else who eats from your garden that all is fine by explaining the chemical qualities of arsenic) maybe it is a better idea to make a set of stairs out of those old pressure treated boards.

Black Locust Beds
Or… you can use naturally rot resistant wood. Black Locust is probably the most rot resistant species you can find locally. White oak is OK. If you have unlimited funds, Cedar and Redwood are sold. You’ll find locust and white oak at Pennsylvania sawmills. Yes, there are still sawmills around. I have 15 year old white oak beds that are just now breaking down. I expect the locust beds to be heirlooms.

Warped bed Board
The bed length is up to you but the longer board the more expensive they get. You can butt ends of individual beds together, as needed, if you have a big garden and want lots of gardening capacity. Bed width… you want to be able to reach across the bed from either side. Three feet wide is good. Wider is a stretch. Literally. If you don’t mnd working from both sides of a bed make them five feet wide. Board width…a full one inch board width will serve you well.

Locust is like concrete and must be pre-drilled in order to fasten end together. Screws are better than nails. Exterior grade screws are best. Plan to assemble you beds immediately after purchasing the boards at a sawmill, unless the boards have been properly dried, otherwise warping will make this impossible later.

How deep should the bed boards be? Six to eight inches is enough. More is a better but the boards will get expensive. Want deeper beds? Make them the identical dimensions and stack them on top of each other.

Hoops and Plastic make a Mini Greenhouse out of a Raised Bed
It is hard to believe that raising your garden surface less than a foot would yield huge benefits…. But it does. You’ll find gardening gets easier and your efforts are more productive. Your fingers are all you need to plant and weed. You will probably find yourself making low covered tunnels to extend the growing season and seeding crops like lettuce in solid beds instead of rows. You’ll make a cold frame instantly by covering a raised bed with a discarded window. You’ll be gardening earlier and later in the season… harvesting lettuce for Thanksgiving… or Christmas!

Ready for a new gardening adventure? Make a couple of raised beds.

Disarded window makes a cold frame out of a raised bed!

For a nice publication on the subject, see this from Missouri xtension.