It is my goal to offer visitors to my website gardening information that will make growing clematis fun as well as successful. That means, when I read advice that can interfere with achieving that aim I feel compelled to bring it to your attention. Unfortunately, in one of my new clematis books, it suggests adding 1 to 2 inches of gravel to the bottom of the pot. Needless to say, I was very concerned about this recommendation so, to save others from making this mistake, I decided to reprint an article I wrote back in the Spring of 2008. After you read it, I am sure you will agree we me when I say “NO GRAVEL!!!”.
There are many gardening myths being circulated today. The reason they continue to endure is because we do things out of habit, belief, or because we were taught that way. Also, writers love to use these adages because they sound “catchy” so there is no reason to question their validity. So, it is no wonder that many gardeners readily accept the myth that adding a layer of gravel to the bottom of their containers facilitates drainage. I too believed the myth until back in 1974, when I was taking a college horticulture class, my professor, John Lenanton, set us straight. Since then I have been horrified to see many well respected sources promoting this potentially dangerous practice. A layer of gravel, rocks, charcoal, shards or similar materials placed in the bottom of a pot impedes drainage and it also decreases the area where the plant’s roots can grow.
Why is a layer of gravel a problem? When we water our plants it soaks into the soil where a certain amount of it is retained by the soil pores and the remainder gradually percolates downward to the depth of the pot where all the soil is filled with water. This saturated water level is called a water table. In a soil profile, the water table (which exists both in containers and in the ground) is the dividing line separating the unsaturated zone from the saturated zone. The region above the water table is the unsaturated zone and is where the plant’s roots grow. The amount of water held by the unsaturated zone (the upper area) depends on the size of the soil particles. In general, the higher the percentage of small sized particles, the higher the water-holding capacity. Despite the small size of particles in potting mix they have a much larger “surface area” than gravel. This large surface area allows the potting mix to hold a greater quantity of water because of its increased surface tension. This is why instead of draining right into the gravel, water perches (or gathers) in the soil just above the gravel. The perched water will not move from the potting soil into the coarse layer of gravel until the potting soil in the container becomes completely saturated. By creating this waterlogged layer where roots are denied oxygen and can rot if water persists for a period of time, this perch water table sadly does exactly the opposite of what the gravel was mistakenly intended to do. Bottom line: Removing the gravel layer increases the space for the roots to grow and allows the potted soil water to drain more quickly because of the elimination of the perched water layer.