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Proper containment is critical to the health of potted plants
By Tony Tomeo
Among the many menial tasks performed in the nursery that I don't miss is "spacing." As the rhododendrons there mature, they must be moved away from each other to maximize sun exposure and allow new growth to extend in all directions. Otherwise, growth is too crowded and only extends upward.
Newly canned rhododendrons are initially placed can to can in order to conserve space, but also to protect the cans from direct sun exposure. (The black vinyl containers in which nursery stock is grown and marketed are known as cans, not pots.) As they grow, rhododendrons may be spaced into areas left vacant by more mature ones that have already been marketed. Spacing accommodates expanding growth but does not adversely expose the cans below. The most mature specimens are eventually able to shade their own cans even if moved away from others.
Shading the cans from direct sun is necessary because the black vinyl can become detrimentally warm if exposed to sunlight for too long. Roots inside are easily cooked if the soil becomes too hot. Even if the soil is not hot enough to cook roots, increased temperatures promote the proliferation of many fungal rot organisms.
Likewise, other plants purchased in nurseries should not be expected to perform in the cans they were grown in unless the cans are protected from sun exposure. Those unable to shade their own cans that are not immediately installed in the garden should be placed in an area where the can is shaded but sun exposure to the foliar canopy is sufficient. Placing recently acquired nursery stock on the north side of a fence or building provides adequate shade to the cans and allows enough sunlight, even if only ambient light, to sustain the stock until installation. Most nursery stock will tolerate considerable shade for at least a few days.
Specimens may be left in their nursery cans if the cans are placed into larger pots or containers, or nestled into shallow shrubbery or deep ground cover. Containers and pots in which plants are grown around the home should not exhibit the same uncomfortable characteristics of the nursery cans.
Although clay is heavy and fragile and wood eventually rots, these are both preferred materials for containers.
All containers require drainage. Pots or urns without drainage holes are only useful for containing other pots or nursery cans if the inner containers are elevated above water that drains from them if any accumulates.
Drainage pans, trays or dishes--as well as undrained outer pots or urns that collect water that drains from containers--should not hold stagnant water long enough for mosquitoes to breed. A constant presence of water is also an indication that irrigation is too generous. Of course, containment of drained water is not necessary for containers on soil.
Regardless of proper containment of drainage water, rot may become a problem under containers on wooden surfaces, such as decks and porches, even if the surface is tiled or painted. Slight elevation of the containers allows the surface below to dry, inhibiting rot. This can be accomplished with the use of platforms with wheels or clay pot "feet."
Unfortunately, most mixed potting soil contains a very generous proportion of peat moss, which decomposes and causes potted plants to settle. As this happens, new soil should not be added to the top of the root system. Instead, the pot should be "stuffed." This involves removing the plant, adding new soil to the container, replacing the plant and adding soil around it to compensate for the wider diameter of the pot higher up.
Flower of the Week: Natal plum
Natal plum, Carissa macrocarpa, grows rapidly to about six feet or more and may be used as a hedge or barrier that anyone unfortunate enough to violate would only go through once. Sharp spines along stems are branched into pairs, which are often branched again to form groups of four spines!
The leathery, two-inch long leaves are a deep, glossy green. The sporadic white flowers resemble those of star jasmine and exhibit a similar fragrance, but are somewhat larger and not as profuse. The red, two-inch long fruit resembles plums, but tastes like sweet cranberries.
Horticulturist Tony Tomeo can be contacted at 408.358.2574 or at LGHORTICULTURE@aol.com.
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Los Gatos, California Since 1881
Natal plums often bear flowers and fruit at the same time, as seen here. They also have sharp spines, which make the shrubs an effective barrier.
