If you are digging a hole to China, burying a beloved family pet, or as past generations of Southerners once did, hiding the family silver from the Yankees, all you need is a shovel and a strong back. Digging a hole to plant a tree or shrub, however, is an entirely different matter.
Forty years ago a neighbor told me, “Put a $5 tree in a $20 hole.” As a reflection of the cost of living, Frank Holzman, in his 2018 book Radical Regenerative Gardening and Farming, now advises, “If you have a $20 tree, plan to put it into a $60 hole,” which he describes as at least 3 feet across and 2 feet deep, with half the soil discarded and replaced with compost. However, a good hole is not quite this simple.
The best planting hole is prepared by a person with some knowledge of plant physiology and soils, the right tools, and a modestly fit frame. We all know plants use carbon dioxide and give off oxygen through photosynthesis. Those remarkable chloroplasts use water and carbon dioxide in the presence of sunlight to produce carbohydrates that feed the world and provide oxygen for us to breathe. However, just as we and other animals use that oxygen for our respiration cycle and burn carbs for energy, plants have the same oxygen requirement for their energy-supplying respiration cycle.
Plants get carbon dioxide for photosynthesis from window-like stomata on the backs of their leaves, but plant roots absorb oxygen for respiration from ambient air moving into pore spaces in the soil. Plants can actually drown if the soil is saturated (all the pore spaces are filled with water and none with oxygen) for too long a time period, varying from one genus to another. Bald cypress can live for hundreds of years in water-laden swamps as their oxygen requirement is different from that of corn, which can’t live more than a few days in a completely saturated field.
So how does that apply to Holzman’s advice? Compost, thoroughly decayed organic matter, has a remarkable ability to hold onto water. We think of water as a neutral substance, but in fact it is dramatically polar — it is called the universal solvent. Compost has many negative charges that bond with the positive hydrogen poles of water molecules. By adding unnatural amounts of compost to a planting hole, you are basically making a bowl of oatmeal for your plant to sit in — a planting zone that remains wet and oxygen deficient.
That being said, compost in warm, moist Southern soils doesn’t last any time at all. My fellow Clemson Extension agent, Tony Melton, has this question on his test for potential Master Gardeners: “What are the three most important amendments for your soil?” The correct answers are, “Organic matter, organic matter, and more organic matter.” In the topsoil-rich Midwest, soils freeze in winter and all macrofauna and microorganisms responsible for breaking down organic matter stop their activity. In South Carolina, those critters slow down in cold weather but never stop eating. I liken it to what happens when your son in college brings his roommates over all the time. Pretty soon your refrigerator and pantry are empty.
A planting hole with half the backfill composed of organic matter is going to actually lose mass and start to sink. That low spot will fill up with water, leaves and debris will settle there, and your tree will end up trying to grow in an unhealthy environment.
If you have decent soil in your yard, have already taken soil tests, and have limed if necessary, Clemson Cooperative Extension suggests that you don’t add compost (or fertilizer) to your planting hole. You want those roots to aggressively move into the surrounding soil and be able to provide all the moisture and nutrient requirements of your new plant as soon as possible. If the planting hole is too comfy, it may retard outward root growth. If you are making a large planting area, you can add up to two parts compost to eight parts soil.
By now you are probably thinking of looking for earthworms in your compost pile and going fishing as you’re no closer to digging a hole than when we started this conversation. Instead, let’s assume you have a 5-gallon container sitting in your driveway with a tree or shrub in it that you received as a gift. Read the tag and find out what conditions this plant wants (full sun, afternoon shade, or other) and how large it is going to be, as well as how tall and how wide. We’re going to assume that this plant, like most, wants a soil that drains well. Scout your yard for potential sites and look up to see if utility wires are going to be a problem with the mature size. If you haven’t done so in the past, call 811 to mark the underground utilities, too.
Gather what you need. I use a mattock to loosen the soil (a mattock is like a pick ax but with a wide flat blade on one end). Mine weighs two and a half pounds, and that is heavy enough. I get the blade sharpened every year at the local hardware store. I also use my long-handled, rounded shovel. Get your pruners and folding saw, too, as you might need them. You want to have a small or folded-up tarp and pine straw or pine bark mulch with small nuggets.
Choose to plant when it hasn’t rained in several days. You should never dig, till, or even mow when the ground is wet as you’ll compact the soil. Remove your plant from the container. If it doesn’t slide out, take your pruners and cut the plastic away. Put the plant on your tarp and find the root flare. The type of tissue that surrounds the stem or trunk of a plant is entirely different from the exterior of roots. There is a flaring, an increase in diameter, at the base of the stem as it transforms into root tissue. This root flare must be planted at or slightly above the soil line. As rooted cuttings are grown and repotted over several years before being sold, extra soil often may build up in the pot. If you don’t see the root flare, you’re going to have to excavate until you find it. Put your plant on its side and start removing soil until you have exposed that area. If any roots have started growing above that transition zone, cut them away.
Examine the rest of the root ball for any circling roots, being sure to look at the bottom of the root ball, and cut them with your pruners or saw through them if the situation is really bad. This happens if plants grow too long in the container. If the roots are just growing together tightly, tease them apart with your fingers. Measure the distance from the root flare to the bottom of the root ball. That’s how deep you should dig your hole.
If your soil tends to hold water, you need to make the planting hole several inches shallower. You’re better off planting high rather than low. The planting hole should be a lot wider than the root ball — at least four or five times. This is the big, wide world that you want those roots to explore, and if you loosen that soil and keep it mulched, it’s much easier for those tiny roots hairs, which are the first to penetrate outward as roots push through the soil.
Remove any turfgrass or weeds from the area you have chosen. Put down a tarp next to where you’re working and start loosening up the area with the mattock. As you get clumps of grass, pick them up and shake the soil loose over the tarp, putting the vegetative matter in another pile. (I guess I should have added a 5-gallon bucket to your list… but don’t you always have to go back to the garage for something?) Once you have a clean surface free from any plant matter, begin to chop a little deeper, being careful not to go below that root flare depth. With your shovel, remove soil to that pre-determined depth at least three times the width of the root ball, putting the soil on the tarp, breaking up any clumps, and mixing it. Finally, put your plant in the hole and lay down the shovel handle to check once more that the root flare is at or above the soil line.
Replace the soil you took out, patting it down gently, not stomping with your foot. At this point, you can use your mattock to make concentric circles in the rest of the area from which you removed the turf, not removing any soil, just cutting through it. This will break up any hardpan and make a smooth roadway for those expanding roots.
Water very slowly to settle everything down. If you have some air pockets that collapse, add more soil from what’s on the tarp as needed. Do not make a well around your tree. Those are almost always run over by zealous lawn mower operators and end up putting extra soil over your carefully measured and created root zone. If you water with the hose on very low, the water will move outward rather than downward, keeping that entire loosened area moist and friable over the next two years, which is the minimum time you will need to supply more irrigation than your sprinkler system puts out.
Now spread pine straw or bark mulch over the entire area, starting 4 or so inches out from the stem or trunk. If you pile mulch up around the stem or trunk, it is like keeping wet socks and tennis shoes on your foot, and with our humid weather you can bet your last dollar you’ll get fungal infections. Plus, those pesky voles love to hide in that material and munch away at your plant’s bark in secret. Mulch suppresses weeds, keeps the root zone moist, and over time slowly adds a healthy amount of decomposed organic matter to the underlying soil. When the mulch settles, it should be about 3 inches or so deep.
At this point, I usually shake several handfuls of compost over the entire mulched area. Pine straw and pine bark are pretty quick to decompose compared to wood mulch, and they don’t grow mushrooms, especially those peculiar stink horns. By top dressing mulch with compost, I start a slow decay process that is going to encourage earthworms and microorganisms that promote healthy plant growth to inhabit this root zone. I’d encourage you do this several times throughout the year. And I didn’t leave out adding fertilizer by mistake. You want to encourage the roots to come into equilibrium with the foliage, and nitrogen fertilizer is going to encourage more leaves.
Wait a year to prune so you have enough photosynthetic material to make the carbohydrates necessary for those roots to take off running. Check on your new plant often; even in winter it will benefit from some extra water unless we’re having an unusually wet season. If you selected this plant for shade, for flowers, or for its form in the landscape, watch how the buds swell and unfurl into leaves or blossoms; perhaps you’ll see insects collecting nectar or pollen. By visiting often you’ll also be Johnny-on-the-spot if some six or four-legged critters start munching on the leaves or if yellowing or other signs of distress appear. Then you can call Clemson’s Home and Garden Information Center at 888-656-9988, and when they ask you questions about the steps you took getting your plant situated in the ground, you’ll be one of the few who gets a 100 on that quiz.
Amanda McNulty is a Clemson Extension agent and the host of the SCETV television show, Making It Grow. She studied ornamental horticulture at Clemson University and served as editor of the Calhoun Literary Society.