Potting Soils

Nancy Delcher
Carroll County Master Gardener Program

Okay, so I lied.......my beautiful Coral Bark Japanese maple died in the pot I put it in! I realized I had to figure out what I was doing wrong. I could have sworn the drainage was good but when I got back from a long vacation, the soil was soggy and the tree was past saving. I could have cried. I wondered how the drainage could be so bad. I looked at the so-called "Tree and Shrub Potting Mix" that I had used - lo and behold, much to my chagrin, right there on the label - "Enhanced to retain moisture"!!! How could I have missed that? I had even mixed it with chicken grit, some home grown compost, and a slow release fertilizer. So I did what I usually do when something goes wrong, I got on the internet and looked around. I knew that Japanese maples don't like wet feet and I wanted to be sure before I went out and spent good money on another disastrous potting mix or even worse, another Japanese maple.

Japanese maples need excellent drainage, acidic soil, and strength fertilizer once in the Spring and again in the summer. Japanese maples are not heavy feeders. So chalk up another thing I did wrong by putting in the slow release fertilizer - these burn the leaves. Fertilizer decreases the intense coloring of the leaves, makes the plant less compact, and doesn't produce as vigorous a root system. They also need to be shaded for the hottest part of the day (no problem here in shadyland). Try to protect your Japanese maples from harsh winds that would dessicate the leaves.

It turns out that it's very hard to find unenhanced potting mix. So I'll have to make my own. I looked for a way to determine if the potting mix would drain properly. The test I found was this: pack a 6 inch pot with your potting soil mix, put the pot in a bucket, pour 2 cups of water through it, and measure the water that drains out - you should get at least 1 cup of water back for the potting mix to be draining well.

There are several formulas for making potting soil that would meet the needs of Japanese maples but all seem to include shredded pine/fir bark. You need the mix to be coarse and not pack down easily - all that dark rich stuff that the stores sell as potting mix is wrong, it holds way too much water and a pot just doesn't drain as well as your garden. Here are several potting soil recipes for good drainage:

1 part coarse or medium grit
1 part shredded pine bark
1 part leaf mold (or compost)

or

1 part coarse sand
1 part soil-less potting medium (no enhancements)
1 part sterile loam soil

or

1 part perlite*
1 part small nuggets of pine or fir bark
1 part sphagnum peat moss

*Remember - there is a difference between vermiculite and perlite...........vermiculite holds water and perlite doesn't - use perlite for aeration, not vermiculite.

Mix your recipe in miniature first and TEST it for drainage as above. When mixing a large quantity, do it in a wheel barrow and make sure everything is moistened equally before placing in a container. The container you choose for a permanent planting like a tree should be thick enough to provide some winter insulation. Glazed pots provide better insulation - they can be coated on the inside with a waterproof coating to prevent moisture from getting in and cracking the pot. If you're going to use a plastic pot, think about wrapping it with an insulating layer of leaves or mulch. Also, roots that are up against the wall of the pot are subject to freezing in the winter and burning in the summer so a pot that's large enough to provide enough room for the roots is essential. Increase the size of the pot every 2 or 3 years by a couple of gallons - or - prune the roots to keep the tree in the same pot.

And keep thinking - good drainage, good drainage, good drainage! I'm getting out the drill to make more holes in the bottom of the pot and I bought a 50 pound bag of large gravel to put in the bottom for even better drainage. I bought the new Coral Bark Maple and if it's still alive in the Spring, I will consider myself lucky! Wish me luck and the patience to really read those #@$#$# labels!

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