How to Keep Your Plants Healthy When You Have to Hold Them Longer
April 19, 2020 
Photo: W. Garrett Owen, Michigan State University
Leading up to the COVID-19 global pandemic, greenhouse growers spent weeks producing crops for spring sales. However, with the coronavirus halting sales in many states, growers are stuck holding and maintaining crops. During this time, according to Michigan State University Greenhouse Outreach Specialist W. Garrett Owen, challenges can arise, such as substrate pH drift. Growers should monitor and adjust (raise or lower) substrate pH to a crop-specific optimal range, thereby preventing the development of nutritional disorders.
In a recent post on the eGro blog, Owen outlines how to handle crops such as zonal geraniums, which prefer a substrate pH range of 5.8 to 6.5. Nutritional disorders can develop if substrate pH drifts above or below this range.
“One nutritional disorder we often see beginning in late March to early April is pH drop in geraniums,” Owen says. “When substrate pH drops below 5.8, iron (Fe) and manganese (Mn) availability and uptake increases. Geranium plants accumulate these immobile nutrients to toxic concentrations in lower leaf tissue, which we often refer to as Fe and Mn toxicity.”
Symptoms of Fe and Mn toxicity in geranium plants begin in the lower leaves, where the initial symptoms can be described as chlorosis or yellowing. These initial symptoms can intensify, and leaves will exhibit chlorotic spotting and interveinal chlorosis. Other symptoms include leaf bronzing, dark brown to black spotting along leaf margins, and marginal leaf necrosis or death. Necrosis often progresses inward between veins and leaves become scaberous (rough) due to tissue death. Leaf drop often occurs, and growers must remove leaves from plants prior to shipping.
Check out the complete eGro blog post here for tips on how to prevent substrate pH drops and subsequent iron and manganese toxicity. The post also includes information on how to conduct a nutritional monitoring procedure and calibrating your pH and electrical conductivity (EC) meter.
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