Building raised beds on asphault

March 19, 2009 7:43 AM

Building a raised garden bed on an asphault driveway. What about drainage?

We're building raised garden beds at the end of the driveway -- right next to the neighbor's converted mother-in-law garage. What, if anything, should we do to ensure good drainage for the beds, and to avoid any possible runoff problems for the neighbor?

A friend suggested punching drainage holes in the asphault. Is that a good idea, or will it just expose our veggies to toxins from the asphault-soaked soil? If we do that, how big/how many holes/what kind of tool to use? Thanks Mefi garden mavens!

posted by ottereroticist to home & garden (8 comments total)

I've never done this, but I can't see runoff being a problem. What, exactly, do you think might be running off?

I'd concentrate on making sure you don't build a bathtub. There will have to be drainage holes along the bottom edges (in the wood, not the asphalt). You also need to make sure that the holes don't get clogged with fine soil particles. I'd build the frame, put down an inch or so of gravel, put down a layer of landscaping fabric on top of that, and finally a mixture of compost and topsoil.

BTW, I'd be concerned that the black asphalt surroundings will make this garden especially hot. It will probably need frequent watering (invest in some drip irrigation and an automatic timer) and cool-season crops like lettuce will bolt sooner, but it will be great for peppers, tomatoes, eggplant, melons and other heat-lovers.
posted by jon1270 at 8:02 AM on March 19

Plan is to build open-bottomed boxes out of lumber and set them on the asphault. I'm concerned that water draining from the open bottoms may pool against the wall of the neighbor's garage.
posted by ottereroticist at 8:09 AM on March 19

Do you get much rainfall where you live? What usually happens to the rain water?
posted by qvtqht at 8:21 AM on March 19

I live in Portland, so yeah. Plus we'll be watering the garden boxes. Not exactly sure what's happening to rainwater back there right now -- it's probably draining off the end of the driveway into what's currently a pile of fill dirt. We're going to move that dirt and use it to fill the boxes. Sounds like a photo might be helpful. I'll post one later today.
posted by ottereroticist at 8:33 AM on March 19

I still doubt runoff will be a problem. Rain that falls here will already be running towards the garage, if the garage is downhill from the garden area, and there's no point in irrigating when the soil is already so saturated that additional water runs right through.
posted by jon1270 at 8:45 AM on March 19

Runoff from the bed should behave about like runoff from rain on your driveway, but you can get out there with a hose and point it at the area where the bed will be and watch for a couple of minutes to get a pretty good answer.

Open-bottom beds will not have overwater issues unless you've caulked the thing to the driveway or otherwise made some sort of seal (which seems like a bad idea on several fronts), because presumably you are using a straight-edged enclosure material that is not going to form a seal on its own with rough asphalt. You may need to water more often, but only enough that you start to see a bit come out from underneath.
posted by Lyn Never at 9:04 AM on March 19

What jon1270 and Lyn Never said. Unless you build a bathtub, it would be as if you piled soil on top of impermeable rock. The water will permeate the soil, hit the rock, make a turn and go wherever gravity takes it.

You can increase the effectiveness of soil drainage by building a mini French drain into your plans for the raised bed.
posted by Cool Papa Bell at 9:22 AM on March 19

Run a sprinkler hose there for a minute and see where the water goes. Does it drain toward their yard or not? It shouldn't really matter because you shouldn't be over-watering the plants anyway. Water from rain will do as it has always done so if there wasn't an issue before there won't be any extra issues now.
posted by JJ86 at 9:40 AM on March 19