Mounding Japanese Maples when you have Clay Soil
The best way to create a raised mound is to first dig a small hole in the clay about 20” wide and 12” deep. Get rid of the clay that came out of the hole. Use wood bark – pine, fir or cedar, compost and/or some potting soil [no fertilizer in it – just plain potting mix] and fill the hole level with the ground. I use a mixture of 70/30 wood bark and potting soil. Then take the pot the maple is in and set it on top of the spot where you just filled in the hole. Push down on the pot until it makes an indentation. Using the indentation as your guide remove about 4” of the soil and bark mix. Then dunk the container and the rootball into the solution to let the bubbles out [just a couple of seconds] and turn the container on its side and slide the rootball out of the pot. Or if the roots are very tight I loosen them before dunking the rootball into the SuperThrive solution [see mixing instructions below] and then place it in the indentation. Have the wood bark and potting mix ready to cover the rootball up to the base of the trunk. Gradually, taper the soil down so that it is level with the surrounding ground to make a mound rather than a volcano. Pack the soil down all around the maple so that when you water it won’t cause the soil to erode. You can step around the trunk of the maple to pack down the soil while at the same time rounding the top of the rootball. Pour the remaining SuperThrive solution around the rootball and pack down the soil again. Add more potting mix if the rootball becomes visible. You need to cover all of the roots, except for the root flare or crown of the trunk next to the soil line. I use a light layer of fir bark as a mulch on top of the soil to keep the moisture from drying out too quickly.
Use a bamboo or other stake and push it through the rootball so that the stake is all the way to the bottom of the hole and it won’t go any further. The placement of the stake should be 1-2” inches from the side of the trunk. Then use gardener’s plastic tape, the kind that stretches, tie the trunk of the maple to the stake about mid-way up the trunk. This will hold the rootball in place during the summer months. When the wind blows on the leaves of the maple it can shake the rootball a little – this movement can break the little feeder roots that are growing from the rootball into the new soil. By staking a maple you are holding it in place so the roots won’t break and therefore get established sooner. You can remove the stake after the leaves have dropped in the fall.
Thoroughly water the maple a week after planting using a small sprinkler or hose drip. You want the water to saturate the soil around the rootball about 12” on each side of the maple if it is a large rootball. For a smaller rootball about 8” on each side should be soaked. If you are in a hot climate watering once a week should be OK, but if you have periods of rain then watering deeply every 10 to 14 days is recommended. You want to fully saturate the soil and then let the roots dry out a little – you don’t want the soil and the roots wet all the time which leads to root rot and will kill your maple.
If you don’t have SuperThrive then you should purchase some online as it is not readily available in local stores. You need a 5-gallon bucket. Mix up 4 gallons of solution – enough to dunk the rootball and have some water left to pour over the soil on top of the rootball after it is in the ground. Use more than the recommended amount. I use 3 tablespoons per gallon or about ¼ oz per gallon. I generally fill the bucket with water to about the 4-gallon mark and add 1 oz of ST to plant two 3-gallon maples or four 1-gallon maples. Using more ST is better than less – you really can’t over do it unless you use the undiluted concentrate. After the initial planting I use prepare the ST solution and give my newly planted maples a drink in late summer and the following spring to make sure the roots get well established. When the roots are sufficiently established the maple is hardier through hot summers and cold winters.