Copper is an element necessary for healthy plant growth, and it is a superhero when it comes to fighting plant disease.
Copper (Cu2+) is a very soft metal. It is also nonreactive and conductive, which is why we use it for water pipes and electrical wires. [I’m not sure why, but rats seem to enjoy chewing on copper pipes and wires.]
How plants use copper
Copper is a micronutrient. While plants only use a tiny amount, copper is critical to many life processes and a tasty harvest. Copper is used by plants in photosynthesis and reproduction. It is a metabolic catalyst that breaks down proteins, increases sugar production, intensifies color, and makes plants taste better. Copper is used to make reproductive enzymes responsible for flowers, fruits, and seeds. Copper also helps roots eat and breathe. Yay, copper!
Sadly, copper can’t always get to where it is needed.
To complicate matters, nutrient deficiencies are not always caused by a simple lack in the soil. Extreme temperatures, insufficient water, and soil compaction are common culprits in nutrient availability. Nutrient deficiencies can also be caused by imbalances with other nutrients. For example, if there is too much phosphorus, which is common in the Bay Area, it is difficult for plants to absorb copper. The only way to really know what your plants are dealing with is to get a soil test from a local, reputable lab. [I wish that those colorful, over-the-counter soil test worked, but they are not accurate enough to be useful. Maybe someday…]
Copper deficiencies appear as chlorosis, twig dieback, and bronzing. It can also cause leaf rolling and curling. If a soil test indicates more copper is needed, be sure to read labels and decide if your soil needs copper that is chelated or not, before adding anything. Chelation is a process that can make more nutrients available to plants, especially in areas with alkaline soil, but too much of a good thing can turn out to be a bad thing. Copper amendments come in different forms. Make sure you get the form your soil needs.
Forms of copper
Beyond pipes, wires, and old pennies, copper can take many forms. In the garden, we generally talk about fixed copper and Bordeaux mixture. Bordeaux sprays consist of copper sulfate, lime, and water. You can make your own Bordeaux spray using materials available at most garden centers. Fixed copper is specially formulated to delay the release of copper ions. When copper ions are “fixed”, they become less soluble in water. This means that, after being sprayed onto leaves and stems, only a little copper is released each time it becomes wet. If a plant receives too much copper all at once, it can be poisoned in a condition called phytotoxicity.
Fixed copper comes in many different forms: copper sulfate, copper oxide, copper hydroxide, and copper oxychloride sulfate. There are also products that link copper ions to fatty acids or other organic molecules.
Copper and pests
We’ve all heard that copper strips repel slugs and snails. This is only partly true. If the strip is wide enough, it will repel snails, but not slugs - and I have no idea why. Of course, if you install a double strip of copper and electrify it, you’ll probably have better luck. Bordeaux mixture or copper sulfate alone can be brushed onto tree trunks to discourage snails. Fixed copper sprays, combined with horticultural oil, applied when pests are in the crawler stage, winter through early summer, can help control scale insects.
Copper as a disease fighting hero
Copper fights diseases by breaking down protein molecules and enzymes within pathogens. There is an astounding number of bacterial and fungal diseases that can be prevented and treated using copper. This is just a partial list:
Generally speaking, copper sprays are applied right after leaf drop and again, just before buds open. If heavy rains occur, additional applications may be needed. Keep in mind, the protection provided by copper only works while the pathogen is on the plant surface. Once infection occurs, copper is ineffective.
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