Q: I like to do a lot of container gardening but have trouble with commercial potting soil. I have tried several brands with little success. I drill drainage holes and feed and water regularly. However, my plants just do not do well. Do you have a recipe for a potting mix that would work well for my containers?
– Donnarae DeWeese
< A: My first thought is that your lack of success, since it is so consistent, might have less to do with your potting soil than it has to do with the amount of light received by your plants. For instance, if you are growing sun-loving, flowering plants in the shade, you simply won't get any blooms. Sun-loving annuals such as petunias and marigolds should get a minimum of four hours of direct light each day, or eight hours of indirect light. An example of indirect light is the dappled light that filters through the branches of tall trees such as Italian stone pine, Shamel ash and eucalyptus. These trees, whether growing directly overhead or across the street, could significantly reduce the amount and quality of light that reaches your plants. Incidentally, shade-loving plants such as impatiens will flower with two hours of direct, or four hours of indirect, light. More often, container plants fail to grow because of overexposure to the sun. Here, we are not just talking about shade plants that get too much sun. Rather, plants of all sun requirements are susceptible to desiccation when grown in containers. The problem is not brought on by sun-sensitivity of shoots and flowers, but rather by what happens to containerized roots. Containers are often placed on concrete patios or decks. Heat reflected from the concrete is directed to the walls of the containers, effectively roasting the roots within. Clay pots, whose walls are quite porous to heat, will warm up faster than plastic pots. For this reason, when watering plants in clay pots, in addition to soaking the soil, it is advisable to hose down the outside walls of the pots as well. During the summer, plants in clay pots will need to be watered more frequently than plants in plastic or poly-resin pots. One way to minimize drying of pots and roots is to position containers within inches of one another; close proximity of plants will increase the humidity around them and their containers. Assuming your plants are getting the proper amount of sun and water, but are still not thriving, the problem might indeed be the soil. Packaged potting mixes may be too rich for the blood of many plants. Ordinary soil is an indispensable ingredient of any potting mix yet packaged mixes may consist entirely of compost, peat moss, pumice and a little sand. If the soil in your garden drains well, add it to the packaged mix or use topsoil, available by the bag in most nurseries, for this purpose. Use soil for one third to one half of the volume of your container mix; the larger the container, the higher the proportion of soil you can comfortably use in the potting mix. Q: Our cactus started to sprout a spear 10 days ago and has been growing about 9 inches a day. Last week it was nearly waist high. Today it is a foot over my head. This plant only does this after 20 years and then dies. The flower it produces at the end can attract wasps. Do you suggest we cut it down before this happens? - Michael Grogan, Canyon Country < A:The plant you are referring to, although it may have leathery leaves tipped with spines, is not a cactus but an agave. The growth rate of the flower stalk of this plant, which can exceed 1 foot per day, is a botanical marvel. I know of no other plant that puts on such an amazing growth spurt. The flower stalk can easily reach a height of 30 feet before it stops growing. Depending on the species, agaves may produce their swan-song flowers - after which they die - as young as age 4 or 5 or, in cases such as yours, as old as 20 years of age. Agave mother plants, by the time they die, will typically have produced a number of offspring alongside - ``pups'' that grow from the roots of the mother plant. Many agaves also have the capacity to reproduce vegetatively on the flower stalk. Miniature agave plants, sometimes in the hundreds, are produced bulb-like along the stalk. Cutting down the flower stalk before it is fully grown will not affect the growth of the plant; its death was set in motion the moment it began to flower. In the case of agaves where offspring are produced along the flower stalk, however, you will lose out on this new crop of young plants.