Answer: Planting where there are lots of tree roots will be a real challenge for your plants - they will have to compete for moisture and nutrients with those tree roots. I have no easy solution for winning the root wars. Growing plants, even grass, under a large greedy tree is both difficult and frustrating.
Amending the soil beneath the tree just encourages aggressive feeder root growth. Before long, new plants are squeezed out, unable to compete with the tree.
Extensive hand digging and rototilling are apt to injure many of the tree's large surface roots. Those wounds can be infected by root-rotting fungi, eventually compromising the tree's health and structural integrity.
Here's a suggestion for growing plants beneath one without risking harm to the tree:
Using closely-knit woven landscape fabric, craft a "bag" that, when sunk into the soil, will prevent tree roots from growing into it. The seams have to be tight, sewn or glued, not just overlapped. An eager tree root could easily insinuate itself between the layers.
Heavy black plastic could work, too, but it would have to be poked with numerous tiny holes to allow for drainage. An ice pick would make the right sized holes.
Face the holes outward (just try puncturing black plastic with an ice pick and you'll see what I mean) to make it easier for water to drain and harder for roots to get in. Concentrate the holes near the bottom of the "bag."
If you know a nurseryman who uses planting bags--the practice is fairly common now--see if he'll part with a few to save the effort of making your own.
Regular rigid plastic nursery pots are not the ideal container for this application. Most are too deep, and the drainage holes would have to be covered to prevent tree roots from invading.
Shade-loving perennials such as hostas and ferns are excellent candidates for planting bags. For a large hosta variety such as "Frances Williams," "Krossa Regal" or sieboldiana, allow one plant to a bag equivalent in volume to a five-gallon pot. Within three seasons the hosta's extensive root system will completely fill the bag. Within five years it will probably have to be divided.
In the same size bag, a gardener could substitute three Christmas or autumn ferns or one small shrub such as Clethra alnifolia "Hummingbird."
When digging holes beneath the tree, position the bags between major roots so they're not disturbed or gouged. If you hit a root, re-cut it cleanly so as to minimize the surface area exposed.
Line the hole with the empty bag, leaving enough material at the top to fold an inch or two over the surface of the soil (yes, you even have to protect against invasion from surface roots). Fill the bag with rich, well-amended topsoil. Without competition, the plant growing in it will get large and full.
One large tree could easily accommodate multiple "bags." Make some a little larger, if you can find space between the roots, and experiment with small groupings of shade-loving perennials.
It's unlikely that you'll ever be able to craft a lush or spectacular shade garden using this desperation technique. However, the planting could certainly be acceptable, and much better than the eyesore of bare mulch.
Speaking of mulch, a two- to three-inch layer of finely-ground bark hides the mechanics of your bag planting, prevents weeds from growing and helps retain moisture. When it comes time to divide, loosen the bag with a spade, grab hold of the flaps and lift the entire root ball. The bag may even be reusable.
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