Sometimes, for no discernible reason, perennial plants and trees don’t grow well, even if the leaves are the color they are supposed to be, without pest damage.
Whether it was planted a few months ago or a few seasons ago, the plant doesn’t get much bigger or wider. Instead of being covered in blooms, it has just a few, or none. It doesn’t look or act the way you thought it would.
Why might this happen? Let’s take a look at a few possibilities.
Root-bound plants come out of a pot from the store with the roots in the same shape as the container. The plant outgrew its pot while waiting for its forever home. If you don’t correct this, the roots will continue to circle the plant or tree. I’ve pulled up plants that have been in the ground for several seasons and the roots are still in the shape of the container. Since it isn’t establishing roots in the soil, it can’t get the nutrients it needs.
Other problems are planting too deeply and burying the root flare of a tree or shrub or planting too shallowly so that the roots aren’t covered. All of these situations prevent the plant from establishing its root structure in the soil.
Small root-bound plants can be soaked in water and gently loosened until the roots come free. If that doesn’t work, use a knife to make vertical slits in the root ball sides and on the bottom. Girdling roots of trees and shrubs can be cut off, leaving the root flare intact. It takes more knowledge and technique to correct a root-bound tree than a plant. If that is intimidating, closely examine the root ball before purchase to avoid problems.
If it’s an entire area of plants that are not thriving, consider allelopathy. Certain plants and trees give off chemicals through their roots, bark, leaves or seeds that prevent other plants from growing nearby.
A common example is the black walnut tree, Juglans nigra. This native hardwood releases a toxic chemical called juglone up to 60 feet away from the trunk that can kill or suppress woody and herbaceous growth. Common landscape choices such as lilac, peonies, hydrangea, privet and yew are highly sensitive to juglone and will wither and die quickly.
There are many plants that don’t mind juglone, however. Monarda, yarrow, day lilies, many ferns and other native plants will be fine and maybe even appreciate that the black walnut keeps other plants away. The Morton Arboretum (mortonarb.org) has extensive lists of trees, shrubs and perennials that will coexist with black walnut.
Ailanthus, or tree of heaven, is an invasive tree from Asia that is found throughout our area, more commonly than black walnut at this point. Its leaf litter has been shown to prevent native species from establishing — one more reason to keep this problematic tree out of your landscape.
Sun, soil, water
Once other issues have been ruled out, review exactly what that plant needs. Look it up in a plant database created by researchers and academics such as at the Missouri Botanical Garden (missouribotanicgarden.org). Always use the scientific (Latin) name when browsing because plants usually have more than one common name and it’s easy to misidentify a plant without it.
Working with the specific needs for hours of sunlight, moisture and soil conditions, assess whether the plant is in the right place. A plant that prefers well-drained soil might not thrive in a heavy clay soil. The roots might sit in water too long. Amending the soil with organic matter can help. Or if you have an area of clay soil where you want to put a garden, let thick layers of wood chips decompose for a few months before planting to improve drainage and nutrients.
Sometimes only a detailed soil analysis will tell the story of a plant’s lack of vigor. There may be concentrations of salt or other chemicals. The nutrients might be insufficient. I’ve found this level of detail isn’t necessary for most landscaping projects. Start with the basics first.