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Question of the Week © 2017 Mike McGrath
Note: Hundreds of Mike's informative articles are available (in alphabetical order!) right here at the Gardens Alive website. To find Mike's answers to your specific garden problem, Click here and find your topic (like Weeds, Worms, Rhododendrons...) in our complete alphabetical archive of Questions of the Week.
Prepping Raised Beds for the Winter with Compost & Leaves
Q. I've heard you talk about adding leaves to raised beds. I want to add compost to my raised beds instead, but I'm leery of buying bulk compost because I fear it may have originally contained treated grass clippings. I found bags of 'raised bed soil' in a big box store; I've copied and included the ingredients portion of the label. Do you think this would be a good addition? I also have some peat moss and thought about adding it to the beds. Would that be OK?
---Henry in Ambler, PA
A. Let's start with the peat moss: No. Although the packaging makes it sound like an all-purpose ingredient, it is not. Peat does lighten soil and improve drainage, but it contains little to no nutrition and is highly acidic. It would be really easy to screw up the pH of a raised bed by mixing one of those giant compressed cubes of the material into the soil.
Side track: "And isn't peat farming bad for the environment?"
That's a complex question. The research I've seen strongly suggests that European peat bogs have been way over-harvested, and their ecosystem is endangered. But the big bricks we see at American garden centers are from Canadian peat bogs, which seem to be very well managed. I personally do use milled peat moss, but only as an ingredient in container soil mixtures and to acidify the soil around acid-hungry plants like blueberries and azaleas.
The old adage comes to mind here: "Just because you have a lot of something doesn't mean you should use it in your garden." Especially if that 'something' radically changes the soil pH—like peat moss or wood ash.
Interestingly, the bagged mix that Henry took a picture of already includes various forms of peat—but with limestone specifically added to adjust the pH. (As I do when I use peat moss in my container mixes.) The other ingredients in Henry's bag ("Henry's Bag": good name for a band!) are composted rice hulls, composted bark and perlite.
Hmmm. That combination sounds more like a potting soil mix for filling containers than 'garden soil'. I didn't see the front of the bag; just the ingredient list, which seems pretty good. But if you're going to buy something like that, use it to fill containers (and if you're buying it at retail get it at a local independent garden center and not one of the megastores that's putting those family garden centers out of business).
Anyway, I basically support using what's in the bag, BUT to get the drainage benefits of the peat and perlite, you'd have to till it into the beds, which would trigger dormant weed seeds to germinate and release nutrients from the bed. Kill a lot of earthworms too, unless the soil is bone dry at tilling time...
Tilling might be worthwhile if garden beds are really 'heavy' (or being newly constructed), but to prevent massive weed woes, you'd have to level the surface afterwards, water the beds and then use a sharp hoe to slice the little heads off of all the new weeds that pop up about two weeks later.
And once you make a raised bed properly you should never have to till it again. So if these beds really need work, go overboard and add lots of soil lightener now so you can leave them undisturbed in the future. But if you're tilling out of sheer habit, stop doing that and just add fresh compost to the surface of the soil each season. (Read our 'no-till' articles for more anti-tilling rants.)
Henry's worry about the possibility of lawn herbicides being in municipal compost is very rational. Persistent lawn herbicides can kill garden plants, even when they've been through a complete composting process. That's why I keep urging big municipal compost facilities to reject grass clippings at the gate, the way they do in Prince William Township just outside of DC.
But those big piles of public compost are a tremendous resource; and luckily, you don't have to worry about herbicide issues—you can easily test a small sample of the compost in question. Fill a container with the compost, plant fresh seeds in it and keep it watered. If the young plants look healthy, there's no issue with herbicides. If the baby plants look stunted, don't use that compost—AND report your results to the people making the compost. (Read our 'bulk compost' articles for lots more detail on this important subject.)
…Which brings us to a related question from Mike in Woolwich Township, South Jersey. He writes: "I'm clearing out the summer vegetable plants in my raised beds and have a wealth of finished compost ready. Should I leave the compost in my piles until spring or should I dig it into the raised beds this fall?"
A. Neither. One of the realities about making your own compost is that it's much more likely to be completely finished in the fall than in the spring, which is fine. Definitely 'harvest' that black gold now; you need to clear out that finished material to make room for the new batch you'll make from this season's shredded fall leaves and spent garden plants.
(Make sure you leave lots of soil attached to the roots of those plants. That soil is 'teeming with microbes' that will kickstart the composting process. Shred the leaves well, chop up the spent plants, mix it all together and you'll make great compost.)
And be sure to use that older, finished material properly by spreading it on top of your raised beds—don't mix it in. (Again, tilling creates weed woes and allows the escape of soil nutrients, especially nitrogen.) Then cover that freshly-applied compost with an inch of freshly-shredded leaves to protect the nutrients over winter.
Rake off the shredded leaves in the spring to help the soil warm up quickly, install your new plants and then put the shredded leaves back in place to prevent weeds and attract earthworms.