Woodland gardens look so enchanting when you see them in garden magazines. The alluring images of moss-carpeted ground, twinkling wildflowers and old, gnarled trunks beckon us to come and wile away a summer afternoon on an old swing set. However, there is one important detail glossed over by those perfect images: gardening under large trees can be exceedingly difficult.
Deep shade is the most obvious obstacle, but there are others that are less apparent. For example, the entrenched root systems of mature trees outcompete the roots of most other plants, leaving your freshly planted understory species starving for moisture and nutrients. The plants might look good for the first season (which is probably when those picture were taken), but after they've exhausted the cushy soil conditions from the potting soil they came in, the root system of the surrounding trees begins to strangle the life out of them.
Heavy leaf litter can cover small plants, further blocking out the sun. Some trees even secrete growth-inhibiting compounds that prevent other plants from becoming established. A phenomenon known as allelopathy, these chemicals can come from fallen leaves, the roots or both.
Look to naturally occurring forest ecosystems for a bit of insight into how to make woodland gardens work. Here are a few tips, gleaned from Mother Nature (as well as from many failed attempts of my own at forest gardens).
Note the degree of shadiness and make sure to plant species that can tolerate it; an evergreen tree with a very dense canopy (such as southern magnolia) located on a north-facing slope will create very dark conditions at ground level compared to a deciduous species with a thin canopy planted on a south-facing slope.
Consider thinning the branches or removing the lower limbs of large trees to let in more light.
Trees with fibrous root systems (such as fir and spruces) or a suckering habit (such as black locust, willows and alders) are more difficult to garden under; tap-rooted species (like oaks, elms and many maples) have fewer roots filling the soil, making it a bit easier for other plants to get established
Some trees are more allelopathic (toxic to other plants) than others — walnuts and eucalyptus are two that are notoriously difficult to plant under. Use containers, even hanging ones, to grow other plants beneath large trees