Important news for gardeners who enjoy flower and vegetable container gardens....
The 'green industry' is responding to the growing outcry to reduce the amount of plastic materials, especially pots of all sizes, that are offered to gardeners. Thus we are increasingly seeing started plants offered in pots created of organic, biodegradable materials - peat, paper pulp, rice straw, processed feather and cattle dung - as well as some strikingly beautiful, high quality ceramic pots.
At the same time, many persons are discovering the happy truth that a great quantity of fresh-to-the-kitchen edibles and herbs can be grown in containers quite well.
With the container options changing, have we learned anything new about growing successfully in containers? The answer is YES! Let's consider first the special growing mix needed for container gardens.
How do plants in containers differ in their needs from plants in the regular garden?
The first major difference is the SOIL (also referred to as growing medium or sterilized soiless mix.) While there might be slight differences between commercially processed and bagged potting soils, they all contain differing percentages of peat (or something similar), soil lighteners like perlite or vermiculite, and possibly some processed and screened compost.
The growing medium for potted plants must be VERY LIGHT and NON-COMPACTING and DRAIN WELL AND EASILY. Soil, taken from the garden is NOT a good choice.
What are container plants' special fertilizing needs?
Because the potting medium is sterile, its primary purpose is to anchor the plant rather than feed it. The container will need to be fed continuously during the growing season. There are many options that work well:
1. Mix the fertilizer granules or the liquid with water following directions for strength and the feeding schedule (once weekly or once every two weeks.)
2. Alternatively, mix up the feeding solution at one half or one quarter strength and use that EVERY time you water.
3. The various potting mixes that advertise that they "feed continuously" for 30 or 60 or 90 days....are they worth the extra cost? Jeff Gillman (see below) comments, "Slow-release fertilizers are the most expensive form (of fertilizer) and are most appropriately used for plants growing in containers, where you do not want to be bothered with reapplying fertilizer often."
Given the cost of adding slow-release pellets or prills or buying the potting soil with the slow-release pellets in it, one might decide to use a potting soil without slow-release prills and simply apply the liquid fertilizer regularly.
But what if you want to plant in a whiskey barrel or other large container?
Suddenly you find yourself looking for a small mountain that no one is using because it seems that the barrel could hold a lot of potting mix.
Any options? How about putting pine cones, pop cans, pot shards, packing peanuts in the bottom third of the container? Is this a good idea? NO!!
Is this a wonderful plan? NO!! It would take needed space away from the roots - not a good idea. It would reduce the amount of potting soil - not a good idea. And help it drain? Absolutely not...and here is why:
Linda Chalker-Scott, Ph.D., an extension urban horticulturalist and associate professor at Washington State University makes the case about water drainage in soils clearly in her writing about garden myths in her book, The Informed Gardener.
She comments that for over 100 years soil scientists have known that water does not move easily through layers of materials of differing texture. Water moving into (or down) inside the pot of finely textured material (potting soil) must become SATURATED before it will begin to move into the next layer of different textured material (pine cones, rocks, pop cans - it makes no difference, it is all bad for the plant roots.) In other words, adding junk to the bottom 1/4 or 1/3 of a pot reduces soil space for the plant roots and increases damaging soil saturation.
Jeff Gillman, Ph.D.,in the Department of Horticultural Science at the University of Minnesota provided an excellent illustration of the movement of water in his book, The Truth About Garden Remedies. Gillman suggests that the inquiring gardener lay a saturated kitchen sponge flat on your hand. It will quit dripping quickly (15 seconds or so). Now turn that saturated sponge 90 degrees to an upright position and water will pour from it.
This demonstrates that the SHAPE of the container has a lot to do with the drainage capacity of the soil + pot. A shallow bowl shaped container will retain much more water than a taller thinner pot. He comments, "Since drainage has everything to do with how much water a container will hold per unit area, the longer container will drain better."
Many of us dump the soil from our containers each fall. The plants go to the compost; the pots are cleaned, left to dry and stored. The soil is put into large garbage cans to be held until next spring when the existing potting mix is mixed with new potting mix, more pertlite or vermiculite is added and the pots are ready to plant.