Just put some pebbles in the bottom of the pot, pour in the potting soil, add the plants and you’re done; right? Well, that’s not exactly how expert gardeners go about it.
Here are a few tips that I and many others practice when dealing with potted plants in their own gardens. Perhaps these techniques will help you be an even better gardener.
Drainage, Drainage, Drainage
If you read two sentences into any plant caresheet, review the cultural information in a reference book or get advice from any expert, the word “drainage” will surely come up. Good drainage simply means that water should move through the soil, from top to bottom and out of the root zone quickly. It doesn’t have anything to do with how often to water, just what should happen to the water once it hits the soil. Tropical plants, succulents, citrus, native plants, camellias and the huge majority of other plants, thirsty or not, want good drainage.
I’ve potted thousands of plants and one thing I always add to my potting soil is extra pumice. I purchase pumice every time I purchase my potting soil. Looking out the window, right now I have three two cubic foot bags of potting soil stacked outside, along with three 1 cubic foot bags of pumice. That’s about the right ratio. When potting almost any plant I blend about one part of the white, rocky, pumice to three parts potting soil. I’ll add even more pumice to plants that want especially good drainage; plants like natives, most succulents, potted bulbs and many Mediterranean species. Since these are many of the species I especially enjoy, I use a lot of pumice.
So, if expert gardeners almost always add pumice to their potting soil, why don’t the potting soil companies makes things easier and just add it for us? Well, they do sort of; it’s called Cactus Mix. But most people who buy potting soil are simply casual gardeners and don’t really understand the importance of drainage. Adding pumice to potting soils would raise the cost of the soil to the point where the average shopper will think they are over paying. Pumice is also about twice the weight of potting soil, so the bag would weigh more, a discouragement to most customers. Finally, when the bag is opened it is going to look different. It’s not the dark, rich brown appearance that most people expect from a bag of potting soil.
In the meantime, I just blend some pumice into the potting soil myself. It’s easy enough.
On the Rocks or Bottom’s Up?
There’s a lingering misconception that adding sand, gravel, stones, broken pottery or another coarse material to the bottom of a pot, before soil is added, is somehow a good idea. It’s not.
As mentioned, drainage is a key to success with potted plants, and gardeners certainly don’t want the bottom portion of a potted plant to become waterlogged, so this sounds at first like a good idea. Unfortunately, it doesn’t work. It is simple hydrology; water does not pass freely from one soil texture to another. In fact, adding anything to the bottom few inches of your pot is a very bad idea, actually reducing your soil drainage, not improving it. It’s ok to put a nylon piece of screen or a pinch of sphagnum moss over the drain hole first to keep the soil in, but then just fill your pot with soil mix from top to bottom – no rocks please.
Refresh and Repot
Soil decomposes. It breaks down in the ground; that’s one of the reasons surface mulching is so important. But soil certainly breaks down in pots as well. That rich, porous, well aerated soil that you started with a couple of years ago will decompose into something much more dense, collapsing the air pockets and slowing down the ever important drainage that we keep talking about. Some potting soils break down faster than others, especially those with a high percentage of forest compost, wood shavings or other organic ingredients. Mineral components, like pumice, coarse sand and perlite, don’t break down very fast. This is another reason that experts and hobbyists add pumice to their potting mixes, extending their lifespan.
To offset decomposition of a soil, potted plants, especially large ones, should be re-potted every once in a while. I periodically go through all my medium to large potted plants, tip them over and check the soil. If the mix has broken down considerably and especially if it has become mushy or slimy I’ll do a re-potting, often back into the same pot. To perform this revitalizing chore, just knock off the outside couple of inches of old potting soil, both on the edge and at the bottom. Dispose of this old decomposing soil and replace the space with a fresh layer of soil mix as described above. Your plants will be revitalized.
Ron Vanderhoff is the Nursery Manager at Roger’s Gardens, Corona del Mar and his profile can be seen by clicking the "About the Author" link below.
Questions from Readers April 30.
My new nectarine has a disease called Peach Leaf Curl. A bought it dormant. Did it have the disease when I bought it and how should I treat it?
Pat, Costa Mesa
This is a very common disease of both peaches and nectarines. You’ll never know how the infection arrived, but sooner or later you would probably see it anyway. Peach Leaf Curl is easily identified by puckered, curled and distorted new spring growth. These leaves will eventually fall off and be replaced by more normal leaves, but the issue will return again next spring and get progressively worse. Treatment is done during the winter dormant season only, so make a note on your calendar. You will be applying a copper ammonium product, usually a brand called Liqui-Cop. Follow label directions and make two applications, one about New Year’s and the other just before the flowers open, usually about mid February.