My Aunt asked me recently about a good weed free mulch for topping her new sheet mulch bed -she doesn’t have a lot of on-site materials so what could she use? As a parent always trying to engage my kids in permaculture I posed this question to my 2 boys –ages 11 and 8. 

Rather than answer me directly they were immediately inspired to create a sheet mulch example out of Lego.  I thought it was a great visual demonstration of all the layers and I decided not to press for an answer.  My question was far less interesting than building this model and they got a load of information from doing it.  Though, when finished, my eldest did tell me to make sure any mulch used was never sprayed with chemicals!

Sheet mulch layers pictured above:

First (and not shown in Lego) is the main soil profile.  Next is the grass or some layer of weeds (no need to mow this area unless they are really tall plants).  If there are trees and shrubs or other large woody plants you know you want planted into the sheet mulch area they should be planted BEFORE the next few steps.  Any earthworks or elevation/grade changes in the terrain should also be performed prior to sheet mulching (i.e. elevated pathways and rainwater harvesting basins). 

Lastly before sheet mulching you’ll want to flag any irrigation system components you’ll want to use after the bed is finished.  It’s a major problem to learn later you’ve been watering under the cardboard unnecessarily or that you can’t get to system without digging up everything!  In the picture below you’ll also notice that 6” or so away from the concrete patio the grass has been removed. This minimizes the grasses ability to grow out from under the cardboard.

The first actual layer we apply in sheet mulching (and key for weed suppression) are potentially seedy manures and fresh compost (kitchen scraps are good) which activate decomposition but never really see the light of day.  After placing down anything that may present a weed issue lay down the cardboard; overlapping the pieces by 4”-8”, at least, to keep out running weeds like Bermuda grass.  A good thick layer is best for areas with pervasive weeds, up to 4” thick if necessary.  We were once told by a colleague of how she was able to remove Himalayan blackberry with very thick layers of cardboard and persistence of reapplication over a year or so.  She tells me the blackberry is a thing of past today –that’s how effective sheet mulching is. 

After the cardboard is laid out water it thoroughly!  Watering promotes decomposition which is the driving force of sheet mulching.  Next apply liberally humus rich compost and/or soil.  Depending on the goal for the new bed native soil can be added in place of or in addition to compost.  After the compost/soil is spread out apply the mulch of your choice.

If you're going with a perennial bed then the woodier the mulch the better (perennials prefer fungally dominant soil) so any trees/shrubs in your yard can be pruned and shredded for mulch. If it's an annual bed then straw mulch (not hay, it’s really seedy) works well mixed with alfalfa (alfalfa promotes beneficial protozoa in the soil –diverse soil microbes is key to healthy soil). If you need to get mulch from off-site you can try to track down local tree trimmers and see if they’ll drop some arbor mulch at your house -but the tree seed factor would need to be evaluated (i.e Privet is great mulch but can be filled with seeds that germinate readily).

Once you have everything layered up you can plant small plants and seeds.  Just uncover the area for the plant, down to the cardboard if necessary making an X in the cardboard to let the roots penetrate easier, place the plant in and back-fill. For seeds just dig them into the soil/compost to the depth they prefer, cover back up and water everything in well.

Sheet mulching in the Fall or Winter is the best time for free moisture which helps decomposition. We’ve done sheet mulch projects where the cardboard is gone in just 6 months with adequate moisture vs. 2 years in very dry areas of the yard.

IMPORTANT TIP: Always remove the tape and/or staples from the cardboard before using it. It’s quite bothersome to garden with bits of tape surfacing all the time and no fun for those of us who like garden with bare hands to get a staple cut.

As to my Aunt’s question of a good weed free mulch? I would first try to find as much mulch as I could within my own yard to shred up.  Things you normally would think of using can be perfect mulch.  Weed trees that need to be removed (without seeds), shrubs that are too tall or wide, trees that need to be thinned all can be easily obtainable mulch. 

If you don’t have a shredder renting or borrowing one is a possibility. I’m fond of neighborhood tool shares too; where neighbors go in on needed equipment then share them around.  You can see if neighbors have extra mulch to share or track down local tree trimmers. Last to choose from are local landscape centers that sell mulch in bulk (from forests in the area if you're lucky) -but then there’s the delivery and gasoline to factor in for transport.  On-site mulch is far better in my opinion but if there's not enough mulch on your site now? You can always plant more for tomorrow!