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Let's Work Up some Window Boxes!

Q. We will be building a window box to mount on a deck rail. How much drainage should be provided? The box will be exposed to whatever rain comes along. Thanks,

    ---Vic in Media, PA
A. The answer is a lot, Vic. Although window boxes do differ from other containers in a few ways (which we will explore shortly), their basic need for excellent drainage is the same.

That means lots of nice sized drain holes in the bottom of the container. I heat up the tip of an old Phillips head screwdriver and use it to poke extra holes into the bottom of plastic boxes (they never seem to come equipped with enough); a power drill for wood. If you're worried about little bits of dirt escaping, line the bottom with window screening or hardware cloth. And make sure the box doesn't sit flush to that deck rail—those holes need to be able to drain.

But that's only half of the good drainage equation; soil mix counts for a lot. So : no garden dirt in your boxes—and none of the cheap, heavy "Potting soil" sold in generic-looking bags at ridiculously low prices at home stores and supermarkets. (I picked one up recently and it felt more like Liquid Brick.) Fill your containers with three-quarters of a loose, light soil-free mix (aka "Professional Mix") and one-quarter compost. Don't use soil-free mixes that contain junk like chemical fertilizers. Mixes enhanced with natural nutrients like worm castings are fine.

And no rocks, pebbles, peanuts or other trash "to facilitate drainage" in the bottom. Your plants' roots want to be able to wiggle into compost and 'soil' all the way down.

Q. We have lovely window boxes on the sunniest side of our row house. But I killed most everything I put in the boxes last year; it all burnt up. (I think my problem was waiting until May to begin planting.) The 'boxes' are wrought iron holders to which we add a liner to hold the soil and plants. What's the best material to use: shaped moss, burlap, wood, clay? We understand that we should use the same non-soil, or light-on-soil mix you recommend for other potted plants—correct? And should we water them like you recommend for a lawn? Or more frequently? Thanks so much,

    ---Sarah in Chestnut Hill
A. Your problem was not timing, Sister Sarah—it was that unrelenting sun cooking your poor plants. In a hot and sunny summer, EVERY plant (except maybe sweet corn) would like some afternoon shade. And your problem is compounded by the fact that most window boxes leave their hinders hanging up in the air, insuring the maximum amount of drying out in the shortest possible time. That makes your choice of liner material for those holders crucial.

Now, nothing looks better than those naturalistic liners made of materials like peat moss, coir or burlap—but nothing dries out faster either. Hanging baskets and window boxes that use these kinds of liners and get lots of sun often needed to be watered more than once a day—a chore that quickly gets tedious and makes even a weekend getaway in the summer an exciting horticultural adventure.

But they can do very well in shady spots. I have several big wrought iron containers like yours, and the ones I've lined with coir or a burlap-like fabric are placed where they'll only get a few hours of sun a day and are filled with things like impatiens and begonias.

The wrought iron holders I have in the sun either have wooden boxes in them to hold the soil and plants in place (the rougher the wood, the better this looks), long plastic window-box like containers, a row of plastic pots, or a combination of those types of containers. My nicest one has a big round plastic pot at either end with an old rough wooden box that was used to ship bottles of Italian wine in the center. I just gathered up a whole bunch of cans, pots and boxes and kept moving them around inside the holder until it looked right.

"Clay" can be almost as tricky as fabric. Like coir, peat and burlap, terra-cotta and other forms of unfinished clay wick their moisture into the air and need more frequent watering than non-porous containers. (They're also heavy and break easily.)

So with your full-sun exposure, make sure you only use plants that can take a lot of bright light (none of my impatiens or begonias!), use only wood, plastic or metal inside your frames, fill those containers with half compost and half soil-free mix (to gain a little more water retention than my usual recipe), and mulch the surface with shredded fall leaves or dried clippings from an herbicide free lawn. Then you should be able to achieve the maximum time between waterings.

Oh, and your plants shouldn't burn to a crisp—that's always a plus.

Q. I'm looking to construct a few window boxes to go around the perimeter of my small deck. I was planning to grow herbs, but my girlfriend wants more "serious" vegetables, like tomatoes, peppers, and eggplant. Is this feasible for the volume of soil I can support in boxes that are 12" deep, 10" wide, and four to six feet long?

    ---Jim, "right here in Philly"
A. In other words, you're making really skinny raised beds. I like it. These boxes sound perfect for herbs, eggplants and peppers. I'd mix some small flowers in there as well, to attract pollinators and 'chust for nice'. You could try a tomato plant, but stick with varieties bred to stay small (look for 'patio' or 'bush' style plants) and be prepared to give up a space that could hold two or three smaller edible bounty producers.

Oh, and be sure to use the boxes to grow lettuce, pansies and spinach in the Spring and Fall; they sound perfect for cool-season salad production!

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