Gardens Throwing Shade Pt III: You've Got This

 

In the previous post, I discussed the various ways the recent history of your yard affected soil quality and your ability to plant woodland species. In this third, and last, post on growing sensitive woodland species in your yard or garden, I will address simple things you can do to make your soil more hospitable to these special plants.

Well above ground, a community of trees we call forest or woodland, is quite different from most yards, even those in neighborhoods full of trees. In many woodlands, certainly the mixed deciduous forest known as the Big Woods, in Minnesota, trees have fewer branches. Why is this? 

Big Woods understory includes shade, fallen leaves, few plants and fewer lower branches.

The main driver is competition for light. Trees will not expend energy growing horizontal branches like those found on a specimen tree in a yard full of sun. The branches they do have are usually striving upward toward sunlight —light that is often the result of a larger tree that came crashing down. It probably took out some neighboring tree branches on the way, reducing lower branches even more. This is not to say there are no lower branches found on mature trees in the woods, but that it is limited by available light. Branches support leaves, and leaves are their to collect light to make food. Yard trees do not look like forest trees because they are exposed to so much light.

The forest canopy is extensive, spreading across the entire woodland to shade the soil beneath and protect it from drying winds and hot sun. In a blow down, where storms knock down trees, several quick-growing species take advantage, beginning to protect the soil beneath once again. The expansive canopy, its layers of leaves above, and understory species below, reduces the impact of heavy rain on soil below. This happens under that big old maple in your yard, too, but it does so without the reinforcing elements of a forest context. 

Rainfall, tapping leaves that slow its descent to the forest floor, is less likely to erode soil and accumulated organic matter. Fallen leaves and wood are not dried by sun or removed by wind, and then decompose. What's more, the sun's heat, but also wind, act as a draw on leaf transpiration, but trees with fewer branches have fewer leaves, and fewer leaves means reduced transpiration —effectively limiting ground moisture from moving back into the atmosphere. Forest soil retains moisture more consistently than yard soil which allows tree roots to grow deeply, less laterally, making it easier for woodland plants to grow under trees. The forest creates its own atmosphere of higher humidity, cooler and a shallower temperature range. Moist, organic soil helps seeds germinate while the moist atmosphere is hospitable to emergent woodland species.

I believe you may now be getting the picture. Common yard soil needs to mimic the soil that sensitive woodland plants require (moist —not wet, organic, and cool). The yard in which these plants are sited must be shaded by trees, protected from harsh wind and sun exposure to increase humidity and regulate temperature.

 

How To Do This?

1. Collect as many fallen leaves as you can, from your yard or others

Tip: Mixed species are best, pine needles fine, but all maple leaves are not ideal

2. Pile leaves in shade

Tip: Shred leaves to halt blow-away and jump-start decomposition

Tip: Mow leaves with a mulching mower with bagger or cart to streamline process

Tip: If leaves aren't shredded, place chicken wire or similar on top to hold in place

3. Keep leaves moist, but not soaking wet —average rainfall is usually enough

Tip: In drought, sprinkle every so often

4. Wait 3 years

Tip: Add new shredded leaves to the same pile each year 

5. Harvest duff-like, partially decomposed leaves from bottom of the pile

Tip: Do not place newly shredded, or whole, leaves thickly onto garden beds

6. Use this duff-like material around sensitive woodland species. 

Tip: No need to incorporate it deeply, just add an inch or two near plants.

The number 5 tip —why is that? 

 

Spreading shredded (or whole) leaves thickly over your garden, then covered in a winter's heavy snow, will create a smothering mat. Organisms attempting to consume/breakdown all that carbon (brown) will rob the soil of nitrogen (green) needed by your plants. Garden soil, probably already compacted, won't benefit much, or at all, from either of the above.

 

Conversely, a large, moist pile of shredded leaves in a shady corner of your yard will not be entirely consumed by earthworms. A large, moist and matted pile will also lack an amount of oxygen that will change the way the material breaks down, leading to the creation of a duff-like substance (the desired produce!). Isolated from the garden, you do not have to worry about how a lot of leaves will affect your plants. There are other factors, like mycorrhizae, that may or may not be present, but this is beyond the scope of the article. 

 

Will it really take 3 years? 

 

Wet or dry years may affect how long it takes for the first year's leaves to turn to duff. Leaves from different tree species may affect timing as well. The number of leaves piled is another factor. My experience in our mostly shaded, upland wooded location is that shredded Bur and Red Oak, Basswood, Sugar Maple and Hickory leaves combined created useful material in 3 years. 

 

How big of a pile? 

 

Depends on how much ground you have available and how many leaves you have access to. Eventually duff breaks down & needs replenishment —just like a compost pile. You will be surprised how small a big pile will become over several years. Eventually, if you do not add more leaves, you'll hardly notice it was there except for that duff-like material where the pile was.

 

What else can I add to the pile?

 

Think of the woods you aim to mimic. What breaks down in that woods -add that. Aside from improbable items like insect & animal carcasses, you can add pine needles, rotted or finely chipped wood, but not too thickly and not cedar or southern yellow pine. If adding chipped wood, layer it thinly (the lasagna method). If you or a neighbor have a chipper, small branches and twigs are a good addition. In spring, add fallen, cracked acorns. Do not add compost materials like vegetable scraps or garden trimmings. We do not want excessive green material added to our carbon, as this will lead to compost, not duff. Of course, avoid weeds.

 

The duff pile is your primary investment in improving conditions for sensitive, woodland plants like many spring ephemerals or species like Blue Cohosh. Consider asking why you want to grow ephemeral species like Trillium or Bloodroot. What is the benefit? How long will they last? If you have yet to grow other, easier woodland species, why might Dutchman’s Breeches succeed?

After all, plants belong to communities of species and we are attempting to create a limited, artificial version of that community. If you want Cutleaf Toothwort and Blue Cohosh, have you first tried White Avens & Lady Fern? If you want Nodding Trillium & Hepatica, consider beginning with PA Sedge & Early Meadow Rue. If you succeed with these easier, readily propagated and available species, you may have that much greater success with cultivating the more challenging woodland species.

Lastly —remember that nearly all upland plants like decent drainage and abhor compaction. And for difficult environments, like a large Norway Maple in the yard with a ton of surface roots or under the eaves on the north side of the house, another approach may be required. Consider limbing up the tree (or taking it down), adding irrigation, yearly dressing with duff, or researching species which can deal with thin, shaded soil and dry conditions during the growing season.

The images below are from my leaf pile. My 2023 year leaves, not shown, were freshly mulch-mowed and piled over the 2022 leaves.
 

These leaves have been piled for a year and have begun to compress into a mat.

These leaves are half-way there, with some 2022 layer visible, top right.

 

Close, but not quite there yet, give it another year.

 

 

Frozen chunk of duff-like layer under a 5 year pile of mixed leaves.

For comparison, look closely at the prairie garden soil, below. Prairie soils can be quite different —from very sandy to silty to gravelly, but what they tend to have in common is lower quantities of organic matter (than forest soil) and good drainage.

Prairie soils are different than forest soil —no duff layer and little organic matter.

Closeup of prairie soil —a mix of sand, small coarse rock and some clay.

 

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