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COASTAL GARDENER
Franklin Laemmlen, Ph.D.
Q:
Purslane (Portulaca oleracea) is taking over my garden and flower beds.
Roundup works, but I am still losing ground. Please suggest ways I can control
this weed.
A:
Purslane is a prolific seed producer. Even a small plant (2-3 inches in diameter)
will have started to produce seeds. The fleshy leaves of purslane also make the
plant resistant to drying out. Therefore, hoeing or pulling the plants out and
leaving them on the ground to dry out often does not work because the plant will
reroot into the soil and keep growing.
You say you have been battling this weed for some time now. If that is the
situation, my guess is that a large seed bank is now present in your garden soil
and flower bed, which will take several years to exhaust, even if you do not let
any plants go to seed from now on.
There are several ways to begin managing the purslane problem. One, use
Roundup, often. Spray often enough, so that no seedlings are allowed to set
seed.This will require a spray about every two weeks, especially during the
summer months. Use very low pressure and a coarse (large droplets) spray so
there is minimal drift. Two, use cultivation. Cultivate the infested areas every
week. Keep the soil disturbed, so the seedlings cannot become established and
plants that are present cannot reroot into the soil. Large plants should be gathered
up and placed in the green waste container. The important message here is that
with herbicide or cultivation seedlings should be killed before they set seed. This
has to be done so that you stop adding new seed to the “seed bank.” Three, cover
the infested areas with black plastic. The warmth and moisture under the plastic
will allow some seeds to germinate, and the seedlings will die due to lack of light.
If you can leave the plastic down permanently or at least for several months, the
purslane problem will be greatly reduced. Four, harvest the purslane and eat it.
Young purslane makes an excellent salad. Again, however, it must be harvested
before it sets seed. Five, use a preemergence weed killer. These compounds
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prevent seed germination or kill the seed immediately as it begins to germinate.
Herbicide products containing oryzalin or isoxaben can be used as preemergence
herbicides. Remember, these products kill seeds, so you must use transplants
(seedlings) in areas that have been treated with preemergent herbicides.
As with all plant protection products, read the label to make sure the product is
registered for the use you wish. Then use the compound according to label
directions for best results.
As you can see by the above discussion, purslane infestations present a difficult
management problem. Frequent and intensive attention to control is the only way
to reduce the problem. Not allowing the plant to reproduce (set seed) is critical to
your ultimate success.
Q:
I have heard that baking soda can be used to control powdery mildew fungus. Is
this true, and if so, what proportions should be used?
A:
Several years ago researchers at Cornell University discovered that several
bicarbonate compounds had fungicidal activity. Sodium, potassium and calcium
bicarbonates were all tested. Of these, the sodium and potassium salts showed the
most promise, with potassium bicarbonate being the most active. The makers of
baking soda (sodium bicarbonate) were reluctant to have their product, which is
commonly used in baking, labeled as a fungicide. However, potassium
bicarbonate has been commercialized as a fungicide under the name Kaligreen®.
This product is registered for powdery mildew control on grapes, cucumbers,
strawberries, tobacco, and roses. The use rate is from 2 to 3 pounds per acre per
100 gallons of water. This rate is equivalent to one-third ounce (9.1 gms) to onehalf ounce (13.6 gms) per one gallon of water.
Send your landscape and garden questions to: The Coastal Gardener, 624-A West
Foster Road, Santa Maria, CA 93455.
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