QUESTION: Last December we lost several of the cedar trees in our yard due to the strong winds. We put all of the branches and foliage through a chipper and now want to use the mulch in our landscape beds. Our neighbor just told us however, that cedar mulch is notorious for killing plants – is this true?

ANSWER: Unfortunately, this is one of those myths that continue to be passed down from one generation of gardeners to the next. There are numerous references in popular garden publications including Web sites that warn against the use of cedar sawdust and mulches citing the leachate as being toxic to both seedling and established plants. Some articles even go so far as to suggest that chemicals released from cedar foliage will cause tip burn on established plants and inhibit seed germination. It’s no wonder there continues to be widespread concern on how safe cedar-based mulches are for landscape plantings.

There is no documented evidence that either leachate or volatile compounds released by cedar foliage is toxic to plants. It is well known however that Cedars, especially Thuja species, have developed chemical weapons against a number of pests and pathogens. Researchers have found that our native Western red cedar, Thuja plicata heartwood contains thujaplicin, a water-soluble tropolone that not only inhibits various bacteria and fungi, but also has anti-tumor activity as well. This anti-microbial activity is most likely responsible for the rot-resistant nature of cedar wood. There is no evidence however that this substance harms plant tissues.

Another component in cedar’s chemical arsenal is thujone. This essential oil is found in both Thuja foliage and other non-thuja species. Best known for its ability to repel clothes moths, thujone and other foliar terpenes also repel, inhibit or kill cockroaches, termites, carpet beetles, Argentine ants and odorous house ants. These compounds are not readily soluble in water but volatilize and become airborne. The lack of solubility also helps to prevent the compound from leaching into aquatic areas.

In summary, despite what you may read and hear, there is no scientific evidence that mulches of cedar sawdust or woodchips will have a negative impact on plant growth.

Whenever a sizeable quantity of sawdust is added to the soil, extra nitrogen must be applied with it. For each cubic yard of sawdust (300 square feet one-inch deep) 3.5 pounds of available nitrogen should be added. This can be supplied by 17 pounds of ammonium sulfate, 11 pounds of ammonium nitrate, or 22 pounds of ammoniated phosphate (16-20-0). For a bushel of sawdust (15 square feet one-inch deep) 3/4-pound of ammonium sulfate, one-half pound ammonium nitrate, or one pound of ammonium phosphate (16-20-20) would furnish the nitrogen needed.

These are fairly liberal amounts of nitrogen. Less could be used if fertilizer is to be used around or under individual trees, shrubs or plants. It is always wise to watch plant growth closely when large quantities of wood mulch has been used. Slow-growing plants with small, pale-green or yellowish leaves usually means that plants need more nitrogen.

QUESTION: We recently applied corn meal gluten to prevent weeds in our garden and so far the weeds seem to be growing better than ever! What’s going on?

ANSWER: Corn meal gluten is well noted for its potential as a pre-emergent herbicide for home garden use. It is a by-product of the wet milling process and is used in cattle and poultry feed and in fish and dog food. It contains 60 percent corn protein that is 10 percent nitrogen. The reports on its effectiveness however have been disappointing, especially here in our coastal area where it frequently rains. In order to completely understand why corn meal gluten doesn’t always meet our weed control expectations, we need to look at how it works. Corn meal gluten controls weeds by inhibiting root formation at the time of seed germination. You must apply the herbicide before the weeds germinate. Any weed that has germinated and formed a root will not be controlled by corn meal gluten.

If the soil surface remains wet during the germination period, affected plants may recover and resume rooting. A short drying period is needed for the death of weed seedlings during the critical time when no root system has been formed. Simply put, moisture dilutes the corn meal gluten to the point that it no longer inhibits root formation and growth. Since corn meal gluten is 10 percent nitrogen, germinating weeds get the added boost of fertilizer. Considering how many days it has rained this spring its no wonder your weeds seem to be growing better than ever.

EDITOR’S NOTE: For answers to local gardening questions, contact Master Gardener Earl Miller at 642-0541 or e-mail hm at eymil@charter.net.