Thursday, September 24, 2009

Rock and roll gardens

Snap, crackle and pop goes my neck as I turn and tip my head side to side and tilt my chin around.  It's been a busy past few days at the computer.

I'm building a new rock garden.  This is another one of those 'what have I gotten myself into?' moments.

I've got two basic techniques that I use to build rock gardens on slopes.  I'll start with a picture of the second kind taken shortly after I finished half of it.  Notice the stepping stones and stairs that come across the lower part of the photo and up the right side already have grass growing around them.  If you're willing to dig a path, edge it, and fill gravel around the steps and stairs of your project, you'll be able to fend off the grass better.  I don't have that kind of landscape.  Ours is a rough and tumble kind of place, more appropriate to weedwacking than real weeding.  Formality here would not only be a little out of place, but difficult to maintain without a lot of money.  The paths might look nice, but would look weird with the weedy, unruly beds that lack mulch because I can't afford to buy 10-15 yards or more of mulch each year.  

Notice how the rocks fit together like puzzle pieces in some places, and have larger gaps in others.  You can plan where plants go by deliberately arranging spaces for them, or you can see what happens when you play around with color and form focused entirely on the rocks.  BTW, those steps may look wobbly, but they are incredibly solid.  That takes practice, patience, strength and skill.  Don't leave exposed overhanging rock unless you have either knowledge or good instincts in terms of stone masonry, engineering, or other applicable stuff.  I used a combination of broken rock (you heard right, I actually broke up rock) tamped under the stones, plus I made sure the vast majority of the weight of those stones were set on solid ground, plus I used very large stones (all heavier than I am) and for the largest overhanging stones, I made sure the next big stone up was resting on it.  It usually took about an hour to set a single overhanging stair.  Dig, test fit, dig, test fit again, dig, test for wobble if the fit looks good, get at least a dozen fist-sized rocks, ram them under the overhang until they break up against each other, test that the rock remains solid when I stand on the very edge.  Repeat with next rock.  (For a rock resting on another rock, you ram the smaller support stones in from the side, not the front.)

Free terrace:  I get big rocks (80-100 pound or bigger if you can get them,) dig holes for them that hold about 20-25% of their bulk, and weed out as many weeds and their roots as I can as I dig.  You don't want to overdig--undisturbed earth that supports topsoil is usually nicely compacted, and you want that solidity to prevent your garden from settling.  If the rocks have a large, irregular feature like a point that's going down, I make sure I dig the hole to accommodate that.  As I dig the holes I often bust up the slope, usually with a mattock, behind where the rock goes so that I have plenty of room, and so that I know there aren't going to be grass roots or blackberry roots back there--they'll be much harder to get rid of later.  Then I place the rocks so that they fit against each other well.  Sometimes I set the smaller rocks so that they're a bit behind the big ones.  

I push dirt into any gaps under the rocks I might find.  I also try to rock the rocks (rock and roll!) to see if there's an attitude that's sturdier than the others, and press dirt around them with my knuckles when I'm satisfied with the position.  When the dirt is level with the surrounding soil on the face and back of the rock, I start tamping with a tamping bar all the way around the base of the rock (except the side where it's against another rock, of course.)  This uses a surprising amount of dirt, and I can often cram dirt under the rock with the tamping bar by going in at an angle.  I also tamp in small stones, fist-sized or smaller, when available for added stability.  Don't overdo the tamping, though.  You can actually tamp the rock out of alignment.  And alternate front and back sides.  When the rock is secure, I start pulling dirt off the slope above it to create a level area.  Sometimes I vary the height of this flat-ish, terraced area, and prevent watering and rain runoff/erosion by inserting rocks at soil level changes.   Then, I start the next row up with the same process, making sure to cut into the slope for the holes for the base rocks, rather than setting them in the soft dirt I just pulled forward to form the terrace.  The idea is to set the next layer so that their weight doesn't add to the load on the lower level--they are going to simply hold back the slope for the next terrace, and will be supported by compacted dirt directly below them (plus whatever you tamp around them.)  This kind of rock garden forms a stair step terrace and can be hard to keep weeded.  If you have the patience, leave it unplanted for a season, and then spray all the weeds that come up.  After planting, maintain with Preen.  Or just do what I do and curse and weed and plant big shrubs in there so that they crowd and shade the weeds out (and mulch.)  This kind of rock garden does best with plants that will drape a bit over the rocks, and it's good to alternate small trees and shrubs with ground covers like snow-in-summer.
Pocket slope (pictured here, year three, with a pile of new stones to extend the garden on the left side):  I call it a pocket slope rock garden because there aren't terraces--the garden follows the existing slope--and I use very little if any local soil.  The soil I add in goes into pockets between the rocks. 

 I get a bunch of landscape fabric, enough to cover the entire slope plus some extra, since it will go into depressions under the rock, which takes extra fabric.  Again, I start at the bottom, using my largest rocks.  I dig a hole for each rock.  Then I take my first rock and I carefully set in and remove that rock repeatedly to make sure it's an excellent fit for the first hole.  I pack dirt firmly around it so that it leaves a good impression, and then lift it out very carefully.  (Sound painful?  It is.)  I put down landscape fabric for the run, with the length running down the holes for the other rocks, and the edge of the fabric set so that most of it won't show on the ground in front of the rocks (some peeking through is okay, even good, as it'll keep down weeds.)  I put in the rock as carefully as I can, in the same position, and start tamping under the landscape fabric under the rock (carefully, as not to tear the fabric,) adding soil underneath the landscape fabric as needed.  If you have a short slope, tamping isn't that important, but for a long, steep slope, this first line is critical and you have to have those rocks in solid--they will be holding a lot of weight.  You might even consider having half the weight of the rock underground for this first course.  For the next rock, you have to lift aside the landscape fabric, and fit the rock to the hole again.  When you've got a reasonable fit, lift the rock out, put down the fabric, set the rock in, and tamp.  Keep going until you run to the end of the line.  For the next row, you want to fold the landscape fabric over to expose the soil above your base rocks.  Make impressions for the next set of rocks, but don't be as worried about getting them in perfect or deep, except for the biggest of them--those will be your foundation stones.  

This kind of rock garden looks best if you vary the size of rock.  Even if you set most of the rocks in pretty shallow and don't bother to tamp much, make sure none of them move if you walk on them.  I usually make cursory impressions for a whole row, lay down the landscape fabric, and then set them in their places, rocking them and adjusting them as needed. Keep an eye on that landscape fabric--you'll want to add the next course of fabric when it looks like the highest rocks on your course will start to overlap onto dirt.  

When you've got the slope covered with rocks, get some clean, weed-seed free dirt.  I prefer a very sandy mixture--it's much easier to weed it, and rock garden plants prefer it.  Fill the gaps between rocks, paying special attention to larger gaps where you can put larger plants--you may even want to lift the stones and get a little soil underneath them, so that your new inhabitants' roots have somewhere to run under for support and still have organic and mineral matter to hold nutrients for the roots.  It may appear at first that you've created a kind of sloping rock wall, and it may even look bland and somewhat ugly.  Don't worry.  Get yourself a bunch of succulents from friends and neighbors (or if you must, the nursery but they really multiply quickly so avoid buying them unless you really have to.)  
Add rock cress, lewisia, compact ornamental grasses, penstemon, super tiny dwarf evergreens (make sure they can handle full sun!) or even bonsai-style trees, and other rock garden faves.  Tight crevasses are great for sedums.   When weeds inevitably appear, they tend to lift right out of that sandy soil, which never compacts.  When grasses take root their roots can't get through the landscape fabric, so even if they get under the rocks, you can lift the rock slightly and yank them out (except for the base rocks--there you've got some work with a weeding fork ahead of you.)  This last photo shows the rock garden after about two years.  These days it's quite overgrown, with the rocks peeking through here and there.

I'm working on a free terrace this time.  My very first rock garden was the pocket kind, though.  It's inundated with strawberries, but only because I'm reluctant to yank them.  When I get around to it, it'll be a cinch to weed.  When doing this stuff, don't work alone--a big rock can roll onto you and leave you injured and pinned, where you'd be vulnerable to all kinds of nasty medical ailments like shock.  Wear latex-dipped gloves--they're clingy, and you'd be surprised how many squished fingers I've avoided because I was able to slip my fingers out of the glove just as the rock started to pinch.  When tamping, wear safety glasses--it's very easy to hit the rock by mistake, and a chip could fly back at you.  

Happy gardening!

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