No, we’re not talking about faulty wiring, or that jar full of pennies.
Copper (and sulfur) have been used as fungicides and bactericides in gardens and agriculture for a really long time. Unlike Bordeaux mixtures, which use highly soluble copper sulfate, fixed copper takes much longer to break down. Each time it gets wet, fixed copper releases just a little more metallic copper onto plants. Copper kills bacteria and fungi by breaking down protein molecules within the organisms.
Shopping for fixed copper
When shopping for fixed copper products, it is not uncommon to see a wide range of active ingredients. To get your money’s worth, look for higher “metallic copper equivalents” (MCE) on the label. MCEs are listed as a percentage by weight, with the most common rating being 8%.
Fixed copper can take any of these forms:
There is also a copper soap (copper octanoate) available that has shown good performance.
How to use fixed copper
The effectiveness of fixed copper against fungal diseases can be improved by adding 1% of a horticultural (not dormant) oil. The oil will help the copper stick to plants longer. It will also reduce damage caused by aphids, mites, and scale. Fixed copper is normally applied using a hand pump spray tank (pictured) or a hose end sprayer.
Fixed copper should be applied while plants are dormant. It is important that you get complete coverage of each and every surface. Once new buds and shoots begin to emerge, phytotoxicity can be avoided by spraying on days when the fixed copper will dry quickly.
Avoid using other foliar (leaf) sprays when applying fixed copper. According to the Michigan State University Extension, “[A]void the use of spray additives such as foliar nutrients, and any surfactants with penetrating characteristics when applying coppers. Fixed copper and lime should not be used with Guthion, Imidan, Sevin, Thiodan, Bayleton, captan, carbamate (Ferbam), syllit, or phosphorus acid-type compounds (Fosphite, ProPhyt, Phostrol, Agri-Fos, Aliette).”
Problems associated with fixed copper
Even though fixed copper is an acceptable organic control, excessive use can raise copper levels in the soil to toxic levels. Once in the soil, copper may leach into ground water. It can also be toxic to aquatic organisms and many animals (but not humans).
As with the use of any chemical in the garden, be sure to read the label completely and follow directions exactly.
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