April 8, 2017
With nearly 90 inches of rainfall recorded on Mount Tamalpais so far this season, the winter of 2016-17 is on track to be one of the wettest on record. Only seven other times in the more than 130 years the Marin Municipal Water District has been keeping records have we had so much rain. And we still have a couple of months of our wet season yet to come!
We’re jubilant that our reservoirs are filled to capacity and that most of the state is out of drought status, but the deluge we received may be more than some homes and gardens could handle. Did you experience torrents of water pouring off your roof and pooling around your treasured shade trees? Puddles spotting your lawn or ponding in your veggie garden? Too much of the wet stuff can bring potential challenges as we head into spring. Steven Swain, Marin and Sonoma County environmental horticulture advisor, thinks we will probably see about the same number of pests and pathogens, but they’ll be different ones.
“We’re likely to have a bumper crop of Phytophthora and Verticillium fungal diseases this year, and perhaps next,” he notes, as “both of these diseases attack stressed plants but require water to infect them.”
So why is too much of this precious stuff a problem? Basic healthy soil is composed of soil particles, air and water. Air in the soil is vital for plant growth providing oxygen for roots and soil organisms. When water fills up all the available air spaces, the soil is said to be saturated, and any additional water collects on the surface in pools and puddles. This standing water limits the available oxygen to plant roots — rather like suffocating them — and making them more vulnerable to attack by disease-causing organisms.
If you reflect on what you observed during the worst of the rain, it may aid you in minimizing future plant problems.
• Soggy lawn? If your feet sink into the soil, stay off it. Marching around a squishy lawn can do damage to the soil and the grasses’ root structure. Lawns submerged for several days in winter and early spring before beginning their active growth phase will generally not suffer serious damage. As the lawn dries out, rake off any excess silt, core aerate and add some fertilizer.
• Muddy vegetable garden? Pets, livestock or other animals in the vicinity? Swain recommends to take care of potential diseases; runoff that may have come into contact with animal waste can contaminate row crops such as leafy greens and root crops.
“While the risk is normally minimal,” he says, “people should still wash veggies if flooding has occurred – even briefly – in the vegetable garden.”
• Standing water around your trees? Otherwise healthy ones exposed to prolonged soil saturation may display symptoms like leaf yellowing, leaf drop, reduced leaf size and shoot growth, and dieback at the crown or top of the tree. If the trees have also suffered from drought, they’ll be more prone to injury. Swain describes the scenario where drought-stressed trees are starving for water and spend fewer of their resources on defensive chemicals to ward off attack. When the water returns, disease-producing organisms recover more quickly than the trees and pathogens like root rots and vascular wilts take advantage. On top of that, the weakened trees are susceptible to “secondary pests,” ones that don’t usually attack healthy trees.