Underplanting Trees in the Yard and on the Parkway
True or false: Mulching a tree circle with wood chips is the best thing for a tree, ecologically.
Actually, a wood chip mulch is the 2nd best thing for a tree. It keeps lawn mowers and string trimmers from damaging the tree bark, it keeps down weeds, it holds in moisture, and it is attractive.
But there’s an even better alternative.
A sedge mulch.
“What in the world is a sedge?” you ask. You see them all the time in woods and probably think that they are grasses. “ Sedges have edges” is a common aphorism, which while not quite true all the time, is true enough that it can be an easy identification of the genus. Its stems are hard and triangular and therefore, have edges; while the stems of grasses are round and hollow.
Carex numbers more than 70 species in Kane County [where I live] . They are the quietly elegant backbone of many moist prairie and woodland communities, and their unprepossessing presence still provides structure for many of our rich, native habitats.
Kane County Wild Plants & Natural Areas
Penn Sedge (Carex pensylvanica), one of the most common woodland sedges, in bloom in April.
Penn Sedge forms colonies of light green, grassy leaves less then 1’ tall, found in patches in oak savannas. Not suprisenly, another name for it is Common Oak Sedge.
6/16/09 Newly planted Penn Sedge under my new Red Oak (Quercus rubra) on my parkway. Prairie Alum Root (Heuchera richardsonii) edges the sidewalk in front.
5/21/11 Wild Geranium (Geranium maculatum) planted among the sedge.
Question: But won’t the sedge plants compete with the tree for moisture? Won’t they require extra watering?
No. Unlike turf grass, sedges have dense, fibrous root systems that hold water. 1/3 of those roots die every year, decomposing, and adding moisture, CO2, and organic matter to the soil.
Let me quote from Gerould Wilhelm and Laura Rericha, “Timberhill Savanna Assessment of Landscape Management” pg. 7, April 2007:
Moisture needs of landscapes in general, whether remnant or de novo, during the growing season, are usually in excess of the amount of rain falling at that time, particularly in continental climates where rainfall is irregular during the growing season. The soil must act as a storehouse that sustains water during both the growing season and dormant period. To make this possible, two requirements must be met: the precipitation that falls must enter the soil in a process called infiltration, and the soil must have a large water-holding capacity that is able to retain much water. Both these requirements require a well aggregated soil (Kohnke and Franzmeier, 1995).
The only way to sustain a balanced level of soil organic matter in such systems in the North Temperate Zone is for graminoid root systems to pervade the rhizosphere, die constantly, and then partially decompose in accordance with a system’s inherent redox environment. …When all this is in balance and stable a soil can be said to be healthy. ( Gerould Wilhelm and Laura Rericha, “Timberhill Savanna Assessment of Landscape Management” pg. 7, April 2007)
Curly-styled Wood Sedge (Carex rosea), growing under a tree in a garden with Wild Geranium (Geranium maculatum). Curly-styled Wood Sedge is common in oak woodlands, forming arched clumps of narrow leaves that grow 8” to 12” tall. Spikes of starlike inflorescences are quite decorative.
Sprengel’s or Long-beaked Sedge (Carex sprengelii) is a showy, clump-forming sedge with narrow, arching leaves that somewhat resemble Prairie Drop Seed in size and look. It is found in shady, moist areas near streams, along trails, and in wooded ravine slopes in nature; in the home garden it grows well in average soil in part shade. It increases by short, creeping rhizomes, forming colonies and by seed. Mine are located between the fence and the edge of the patio; they are now embedded within the flagstones, as well. I’ve read that it is often associated with calcareous rocks and soils. I’ve found new plants at some distance from where I first planted the sedge –I would guess that ants are spreading the seeds. Carex sprengelii flowers prettily in May; as the seeds form in summer, the flower stems arch gracefully.(As I look out my window this morning into my patio, the blades are starting to green up.)
Common Wood Sedge (Carex blanda) with Virginia Bluebells (Mertensia virginiana), Jacob’s Ladder (Polemonium reptans), and Red Trillium (Trillium recurvatum) growing under a Hackberry (Celtis occidentalis) at the Chicago Botanic Garden.
Common Wood Sedge is–well–common. Its coarse, evergreen leaves are often found in lawns. When mown over with the lawn mower constantly the blades stretch out horizontally over the lawn in a flat 6”-12” circle. If grown in a woodland or garden, however, they are grow upright and are quite attractive. (See above photograph)
Other Charming and Useful Woodland Sedges
Partial to full shade, mesic to dry-mesic soil
Carex gracillima — Purple-sheathed Graceful Sedge
Carex jamesii –– Grass Sedge
Carex normalis — Spreading Oval Sedge
Question: But isn’t it true that sedges take up a lot of water; then through transpiration, 90% of it is lost to the atmosphere, thereby depriving the tree of that moisture?
As far as transpiration goes, I would like to quote Dr. Wilhelm again: The green growth of the season from the tufts or bunches of prairie grass, moves cool water from the below-ground rhizosphere up to the leaves for transpiration, which keeps the leaves relatively cool. At night, water from the humid air condenses on these cool surfaces and moves down into the ground, which sustains soil moisture–which both waters the system and keeps the oxidation rate lower than would otherwise be the case. (Gerould Wilhelm, “The Realities of CO2:Seeing through the Smog of Rhetoric and Politics”).
Have you ever walked outside on the grass in the summer? It’s always wet, isn’t it?
You can find Dr. Wilhelm’s research papers at this link:
To hear more on this topic, I just ran across a videotape last week of a keynote speech given by Dr. Wilhelm at “The 2007 Living with Lakes Seminars” sponsored by the Valparaiso Chain of Lakes Watershed Group and the Duneland Sierra Club on May 19th, 2007.
You can watch it here:http://www.youtube.com/watch?v=KwspPmnWjl8