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How to best dig through frozen ground to put in raised flower beds?
March 15, 2014 12:22 PM   Subscribe

We'd like build and replace some raised garden beds for a school in Chicago. We've had an unseasonably cold winter and the ground may still be frozen. We have the last weekend in March dedicated to this project. How much can we reasonably dig out and how? What are some rental tools to look into? How long would it take to thaw the ground with fires and heat lamps?
posted by jadegenie to Home & Garden (11 answers total) 1 user marked this as a favorite
 
Is there any possibility of rescheduling? You won't be able to dig down more than a couple of inches into the actual ground. The already existing raised beds may be more thawed, but not predictably.
posted by sciencegeek at 12:41 PM on March 15


You might have some luck if you cover the area with heavy black plastic between now and then, but it kind of depends on how much sun (and not freezing) you get in the next two weeks.
posted by Lyn Never at 12:45 PM on March 15


A rented electric demolition hammer will help with frozen soil (or clay, or caliche, or sandstone). They're fun.

You might be frozen to a depth of 12 inches, so it'll be slow going, no more than 3 cubic yards per 8 hour day (for an able-bodied adult male), and less for the rest of us.

The cheapest soil warming I can think of would be to cover the area you intend to excavate, with a generous margin, with reflective foil insulation (reflectrix or equivalent) during the nights and when the temperatures are below freezing, and uncovering it without fail when the sun is shining on it or when temperatures are above freezing. When you cover it, make sure that the edges of the insulation are secure well enough so air does not circulate under it.

Computing how long it would take to thaw the soil would require that you know the depth to which it was frozen (which you could discover with a pick and shovel and exploratory excavation), the measurements of the bed you intend to excavate, the depth to which to intend to excavate, your available equipment, and your budget.
posted by the Real Dan at 1:38 PM on March 15 [1 favorite]


My thought was to somehow use water to do the thawing; Google gives me a hit in a book called Frozen Ground Engineering that confirms this is a thing - drilling holes and circulating hot or even cold water to achieve the thawing. Though it says it works best with "relatively free-draining granular soils", and it looks like that book is a civil engineering text, so maybe not quite the same application you've got.
posted by XMLicious at 1:40 PM on March 15


Shovel off the snow and use black plastic. You could also build a cold frame (think low greenhouse) by building the raised beds and topping them off with plexiglass or clear plastic. Even on cold days, the sun is powerful. Find some Master Gardeners through the Cooperative Extension Service, and talk to them about typical dates when the soil can be worked in your area. Even with such a wretchedly cold winter, the soil may only be a week or so behind schedule.
posted by theora55 at 1:44 PM on March 15


How big an area (how many beds) are you are looking to unfreeze? If it's only the beds themselves then you could go old skool: in days of yore, gravediggers would at times* build fires over the bit of frozen land they wished to put a grave in, cap the smoldering embers with stone or a hardwood cover, and let it sit overnight. The ground was usually diggable by morning.

Several bags of charcoal and an old bath tub / refrigerator crate could serve in this process.

That said, why dig at all to make new raised beds? Raised beds can be built on top of the ground, no? A bunch of four-by-fours and a few bags of potting soil and you're all set.

If you need to remove existing raised beds, then renting a Bobcat backhoe would be the quickest solution.

* Of course, traditionally, most gravediggers in northern climes just waited for spring.
posted by jammy at 2:50 PM on March 15 [2 favorites]


Raised beds shouldn't need any digging out, provided the beds themselves are six inches or more deep. Only the corner posts are going to need to go down into the ground, and then maybe six or eight inches for those.

We have very heavy clay where I live, so I did dig maybe three or four inches into that, so that I could add a lot of organic matter. With a better soil I probably wouldn't have bothered.

The roots of any vegetables will tend to break up the soil anyway, so there's that. Also, nobody is going to be planting anything until the last frost is gone, so why not build the beds now and do the digging to loosen up the soil inside later on?
posted by pipeski at 3:08 PM on March 15 [1 favorite]


As a fellow Chicagoan, I totally feel your pain!

It does look like in the next two weeks the day temperatures should be above freezing during the day, which should help (and are barely going to dip below freezing at night) At least the top layers around where I am are more completely muddy than frozen. (I almost got stuck in the mud yesterday while walking *grr*)

As you are on school grounds, fires are probably completely off limits.

I'd go out a couple days before hand, and try to put in 1 post to see how far deep you get before renting any equipment.

Good Luck!
posted by AlexiaSky at 3:10 PM on March 15


If you contact the Cook County Extension (as per theora55 above), they can give you some tips.

But yeah, with raised beds, you don't really have to worry about the thaw below ... by the time plants' roots are that deep, the ground will be plenty thawed underneath. (Also the soil is going to be WET AS FUCK with all the extra snowmelt and early spring showers, so miserable and heavy to dig in any case. If you've got to dig, plan for that. It's going to super-suck.)

Also, contact the buildings & grounds people for the district in question (or go through the principal of the school to do so); they have landscaping tools and experience and they may be able to rototill for you in advance or whatever, so you just have to build the beds. (In my district, we are always willing to send out some guys to pre-prep for community-garden-type-things, and the guys like doing it.)
posted by Eyebrows McGee at 5:41 PM on March 15


I agree with the don't-bother-digging folks. Rough out your frames, then put cardboard or several layers of newspaper over the grass before fixing the frame in place and adding soil. The paper layer will smother any weeds and grass, and it will eventually just compost.
posted by Camofrog at 3:25 PM on March 16


Thank you for all your answers! I knew AskMe would pull through. We won't be able to reschedule, but you're right! We probably won't need to dig too far down. In the mean time, we'll just use brute force with pickaxes and sledgehammers to break up the soil and remove what we don't need. It should work out.
posted by jadegenie at 5:26 PM on March 20


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