The Chemical Warfare of Oaks in your yard
I don’t know about you, but I have a hard time growing around oaks. In my yard there are thee oaks that obscure the sun. There are two two monstrous Live Oaks (Quercus virginiana) in the front yard, and in the back yard there is an ever expanding Red Oak (Quercus rubra).
Each year these three trees dump 20,000+ acorns across the property, and shed 40 leaf bags worth of litter onto the yard. On top of all that, those acorns can turn into saplings rather quickly if they don’t get raked up. In spite of this, I tolerate these trees for the wondrous deep-shade that they extend across my yard during the heat stroke of summer. However, the sub canopy under these trees is barren wasteland. I’ve tried exotic shade grasses, ferns, hollys, mints, and the only luck I that I’ve had is with the summer-proof Inland Sea Oats (Chasmanthium latifolium).
I feel like I’m in a constant battle with these trees and their shade. Its seems like nothing can grow under them no matter how much I mulch and water. This is my second year growing under these oak and with such little success, I thought that I was just growing a black thumb. This was until I started to see the same symptoms of the barren sub-canopy in my vegetable beds.
This year I put in two raised beds in full sun. With the help of the whole family, we mixed together brand new soil with the freshest of compost, and micro nutrients in beds that were well away from the shade of the oaks. In these new vegetable beds I had planted some seed-grown tomatoes in one row, next to several rows of mint. I planted so much mint because I heard that mints were great at attracting pollinators and tomatoes are pollinated by the buzzing of bees on the flowers.
The tomatoes grew pretty well until they didn’t. Eventually, they just ended up looking like leggy green weeds with the tiny sad flowers. Since it looked like these tomatoes were ill, I pulled them out and replaced them with some larger store-bought plants. It wasn’t long before the replacement tomatoes began to look like the former tomatoes. Why was I growing leggy green “weed-matoes” while the mints were looking amazingly? In a word, allelopathy.
“Allelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the germination, growth, survival, and reproduction of other organisms. These biochemicals are known as allelochemicals and can have beneficial (positive allelopathy) or detrimental (negative allelopathy) effects on the target organisms and the community.”Wikipedia
Could it be that the oaks in my yard were committing the same plant-on-plant chemicide as the mint was on tomatoes? How could these oak affect the whole yard? It turns out that all those acorns and leaf litter can release allelochemicals into the soil. The longer these bio-litters accumulate in the yard, the more they get broken down into the soil, and therefore release more allelochemicals.
I’ve read some articles about different varieties of oaks causing more noticeable allelopathic effects than others. This is because some contain varying levels of “phenolic acids” and “total phenolics” in their soil litter (acorns, root exudates, and other debris from the oak tree). Live oaks have surface clinging roots which are well known to affect the surface area surrounding the tree too. And it just so happens that my front yard has two of these chemo-releasing culprits.
I think oaks as a species are just running their own HOA. They are very particular about who is allowed to grow underneath their canopy. So every plant isn’t affected the same by the oak’s allelochemicals. The high tannins that are released as the tree litter breaks down into the soil, can actually acidify the top layer of the soil making it rather delicious for the acidic loving Holly, Rhododendron, and Rose families. Keeping in mind that you plant appropriate varieties that can still tolerate the deep shade under oaks. I actually planted a very fragrant Daphne odora shrub under one of the live oaks. This shrub has doubled in size every year and blooms religiously. Yet, I haven’t noticed any effects of the live oak’s roots.
But Daphne isn’t the only survivor allowed under my oak. Here is a list of plants that I have had good luck with under my oaks in Zone 8, dry shade, calcium-rich, heavy black clay soil–so its a short list.
Inland Sea oats (Chasmanthium latifolium) I have been growing more clumps each year. I just can’t get enough of this durable native grass.
Turks Caps (Malvaviscus arboreus var. drummondii) the red variety is widely available and fairly durable, and the fruit is edible. However I tend to favor the pink variety more, even though there’s no fruit. There’s just a magical about the pale pink flowers growing in the shade above a deep green mass. There is also a yellow variety, that I’ve just never had any luck with.
Dwarf Palmetto (Sabal minor) This was recommended by wildflower.org and I have seen this growing wild in & under live oak all across the humid south. I haven’t tried growing this palm, but I’m actively looking for specimens
Yaupon (Ilex vomitoria) I’m constantly pulling saplings out from the leaflitter under my oaks. I don’t like their growth habit, so I weed them out. Notice the latin last name on this plant–definitely not edible.
Coral Honeysuckle (Lonicera sempervirens) Can you ever have too much of this very threatened native?
Spider Lilies (Hymenocallis liriosme) although they like a bit more light inorder to flower fully
Penstemons If planted in the edge of the drip line under the tree, these perrenials are really strong bloomer, and reseaders. Plus there are so many varieties across the U.S. that surely you can grow one of these fantastic flowering plants.
Horney Goat Weed (Epimediums) The two specimens that acquired from Plant Delights, were planted this spring. They bloomed very well for me and seemed to keep blooming with the rains. However, I won’t know until next spring if these plants are affected by the oaks in the long term.
South American Rain Lilies (Zephyranthes) Really good luck with this family. I have minis to large varieties hidden throughout my front yard which get beautiful morning light. #springephemerals
Lenten Rose (Helleborus) From my growing experience, I think these plant are barely a zone 8 plant. They get pretty wimpy looking during the summer, but once the Fall drops the temperatures, they perk right up and start to form blossoms. The main reason I keep trying with these plants is because they bloom well into winter and finish blooming by spring. Unaffected by any frost or ice on their blooms, it’s nice to see these in full bloom, surrounded by the occasional Texas snow.
If you know of any other plants that I haven’t mentioned, please email me your successful suggestions to firstname.lastname@example.org
More articles about Oak and Allelopathy:
- Mr. Smarty Pants talks about allelopathy in oaks
- All about Oak in Texas
- A study of phenolic acids and total phenolics in soils around oak forests
- Understanding Allelopathy as an alternative for chemical weed suppression in food crops
- Plant friends not enemies, a planting guide for your vegetables and herbs
- Common allelopathic plants