What to plant underneath your landscape trees
Story and Photos by Rebecca Stoner Kirts
Great things can happen in the garden – whether it’s because you have a good plan, an accident, or maybe just luck. My gardens are excellent examples of all these. I have learned as much from my mistakes as my successes. Planting under trees falls into the lucky category.
My friends tease me by telling everyone to beware of Becky’s passalong plants. I love all plants and try to find a perfect growing spot for ones that other people refuse to plant because they are, shall we say, a little on the “aggressive” side.
This obsession with finding a perfect place for every plant to thrive is how I first discovered the art of landscaping under trees. Mint (Mentha spp.) is a perfect example. Even the most inexperienced gardener knows that mint in a garden bed is a major gardening NO. I read somewhere that, “Growing mint is like raising a teenager. Easy to have but impossible to control.” The suggested solution was to plant mint in a sunken pot – been there, done that, and it did not work.
My “aha” moment came when I was visiting a garden and noticed that they had planted mint under some towering trees. I now have lots of mints growing under my awesome trees. Since they are very tall and have very few lower limbs, the mint receives adequate water, sun, and space. The bonus: my husband enthusiastically mows around the trees, which keeps the mint in bounds. The mint “barrier” keeps the tree safe, and my abundance of mint has ramped up my popularity at Kentucky Derby time.
So many of my “wild” passalong plants have found great new homes. I once had lily of the valley (Convallaria majalis) that was not behaving correctly in my garden beds, but it now fills the entire area under my gigantic Siberian elm (Ulmus pumila). When they are in bloom, their sweet fragrance scents the air. The remainder of the growing season, they provide a carpet of green, no mulch needed. I have begun incorporating spring-blooming bulbs into this area, since lily of the valley doesn’t appear until later in spring. It is quite a beautiful sight.
Some people struggle trying to grow grass under trees and are looking for a good alternative. They may also know that mowing too close around the tree is not great for the health of the tree or the mower.
Others may see the space under the canopy of the trees as a perfect opportunity for a flowerbed that is aesthetically pleasing and beneficial for the tree as well.
Another reason I plant under my trees is to save money. It’s a pretty large area I need to cover and mulching it all would be require a significant amount of money and time, so I needed to come up with an alternative. Planting under the trees and covering the area with green mulch was a fantastic option.
But first things first: The health of the tree is the most critical factor. Study the needs of the tree you want to plant under. Some trees are much more sensitive to this than others.
If you need to remove any turfgrass around the tree, try smothering it with cardboard. Please don’t aggressively dig it out! Tree roots are easily damaged. And remember that small roots can spread as far out as the drip line.
After the area is clean and ready to go, it is time to plant – using small plants to minimize the size of planting holes. It may take longer to cover the area, but remember – the health of the tree is what’s important and you don’t disturb the root system. I prefer to plant perennials rather than annuals. That way you aren’t disturbing the area every growing season. However, self-sowing annuals are a great option.
Be cautious with any roots you encounter. I start about 1 foot out from the base of the tree. If I encounter a mass of roots, I just move over and dig another hole. I use organic mulch around the plants and trees until the plants have spread enough to make mulching unnecessary.
Note of the degree of shade under the canopy throughout the year. Different growth habits create varying degrees of shade. Under deciduous trees, I often plant bulbs for early spring blooms and then a perennial ground cover for summer. If the tree has a dense leaf drop in the fall, it is advisable to pick up the leaves. I shred them and put them in the compost pile.
Remember to water! The tree and plants are competing for all available water and nutrition. You have to double up on everything. Once the area is established, you can water less often, but keep a close watch.
I enjoy planting bulbs under trees. I dig individual holes, but group bulbs by type. I have a fantastic mass of daffodils (Narcissus spp.) under the big maple tree (Acer spp.) in the front yard. English ivy (Hedera helix) also covers the entire area. After the bulbs have died back, I keep the ivy in bounds with a string trimmer and I never allow it to climb the trunk of the tree.
It is essential to do your homework before choosing plants for these areas. I have made a list in the sidebar of some plants that seem to thrive in this environment.
Remember these critical points as you decide what to plant under your trees:
1. Do trim up any low branches. Don’t get carried away and make sure you understand how to make good pruning cuts. I prefer using low-growing plants under the trees. Taller plants can interfere with the tree limbs, plus I do not have to prune the trees as much.
2. Mulch judiciously. Never “volcano” mulch. Keep mulch at least 1 foot away from the trunk of the tree. Never build a raised bed around the perimeter of a tree. It is a false notion that adding tons of compost and fresh soil will make for a lush garden under the tree. It will smother the surface roots and you will likely end up with a showy raised bed, but a struggling tree. A good rule of thumb is to maintain the existing grade around the tree.
3. Choose your plants wisely. It can be a harsh, dry environment under the canopy of a large tree. A small amount of organic compost surrounding the plants and frequent watering will help.
By choosing the right plants, following some fundamental gardening skills, you can have vibrant green mulch and a winning scenario for the tree, the plants, and the gardener.
BECKY’S BEST BETS
Some of my favorites for planting underneath my gentle giants
Hosta – Talk about incredible varieties. Foliage varies from gold-trimmed, to blue, even white. Varieties with chartreuse foliage are perfect for lighting up what can be a dark area. They are also great combined with other shade lovers.
Foamflower (Tiarella cordifolia) – Its interesting maple-like leaves shoot up a spiky flower. ‘Pink Skyrocket’ is a nice variety. It spreads quickly and is enjoyed by pollinators as well.
Canada wild ginger (Asarum canadense) – You can’t go wrong with this easy-growing perennial ground cover. The heart-shaped leaves spread rapidly to fill in blank spaces.
Ferns – When I purchased my property some 30 years ago, there was a fantastic fern already spreading under some of the trees. I am not sure of the name, but I mix in Japanese painted ferns (Athyrium niponicum), maidenhair ferns (Adiantum pedatum), and others to create a stunning display that requires little care and no mulch.
Sweet woodruff (Galium odoratum) – A shady woodland herb that has terrific tiny leaves and small white flowers. It is infused in white wine to make “May wine.” To me, it smells faintly of freshly mown hay.
Cranesbill (Geranium spp.) – These form a lovely carpet of leaves and small blooms. They quickly and efficiently cover large spaces.
Black-eyed Susan (Rudbeckia hirta) – Great for sunnier areas under trees and spreads quickly. My father’s favorite, I have continued the tradition and discovered that when combined with coneflowers (Echinacea spp.), they won’t grow quite as tall.
I also recommend combining native shade-loving plants, such as Virginia bluebells (Mertensia virginica), lady’s mantle (Alchemilla mollis), coral bells (Heuchera sangiunea), hardy begonias (B. grandis), and any of the many varieties of Brunnera.