Erin Dicaprio, Associate Professor of Cooperative Extension, Department of Food Science and Technology, University of California Davis has summarized information and resources for assessing crop food safety after a flooding event.  There are links to the Leafy Green Marketing Agreement Flood fact sheet and also a presentation made by Trevor Suslow, emeritus UC Cooperative Extension Specialist.  Please follow the link below.

What Can Happen With Too Much Rain?  And Mud?

Rain is wonderful stuff. If it comes and washes the accumulated salts of the last several years out of the root zones of citrus and avocado, that's a good thing. But what happens if there is a little too much of the good stuff? In the winter of 2005, Venture got over 40” of rain, which is 200% of what is normal. The last time big rains occurred prior to that was in the winter of 1997-98. That year the rains were evenly spaced on almost a weekly basis through the winter and into the late spring and over 50" fell. That year we had major problems with both citrus and avocados collapsing from asphyxiation. The same occurred in 2005, but not so pronounced.

This winter we have had a lot more rain than we normally see and in some young trees with poorly developed root systems, we have seen some collapse.from asphyxiation. Avocados tend to be more susceptible than citrus, and some rootstocks more than others. We have also had some trees buried in mud slides, which can also lead to asphyxiation.

Asphyxiation is a physiological problem that may affect certain branches, whole limbs or the entire tree. Leaves wilt and may fall, the fruit withers and drops and the branches die back to a greater or lesser extent. The condition develops so rapidly that it may be regarded as a form of collapse. Usually, the larger stems and branches remain alive, and after a time, vigorous new growth is put out so that the tree tends to recover. Young trees can be harder hit, but sunburn damage from lack of leaves may be more of a problem.

Asphyxiation is related to the air and water conditions of the soil. The trouble appears mainly in fine-textured or shallow soils with impervious sub-soils. In 1997-98, this even occurred on slopes with normally good drainage because the rains were so frequent. When such soils are over-irrigated or wetted by rains, the water displaces the soil oxygen. The smaller roots die when deprived of oxygen. When the stress of water shortage develops, the impaired roots are unable to supply water to the leaves rapidly enough and the tree collapses. The condition is accentuated when rainy weather is followed by winds or warm conditions. These are exactly the conditions we have seen in the last two weeks, hence some of the problems in young orchards on heavier soils.

It doesn't take standing water to have asphyxiation occur.

Canopy treatment in less severe instances of asphyxiation consists of cutting back the dead branches to live wood. If leaf drop has been excessive, the tree should be whitewashed to prevent sunburn. Fruit, if mature should be harvested as soon as possible to prevent loss. In the case of young trees, less than two years of age, recovery sometimes does not occur, and replanting should be considered if vigorous regrowth does not occur by July. As soon as defoliation is evident, whitewashing should be done to protect them to give them a chance for recovery.

Asphyxiation can be reduced by proper planting and grading. If an impervious layer is identified, it should be ripped prior to planting. The field should be graded so that water has somewhere to run off the field during high rainfall years. Heavier soils might require planting on berms or mounds so that the crown roots have a better chance of being aerated.

Hindsight is always great. Post-plant, if an impervious layer can be identified and is shallow enough to break through, ripping alongside the tree or drilling 4-6 inch post holes at the corners of the tree canopy can improve drainage. It is important that the ripper blade or auger gets below the impervious layer for this technique to be effective. If there is a thick layer of mulch reducing soil evaporation, pulling it back to allow the sun to help dry it out faster will help. It's not a lot of work with small trees, but big time work if it's big trees with thick mulch.

Asphyxiated tree that has been whitewashed

In the case of trees being buried, especially covering the bud union, they should be dug out as soon as practical.They need to be dug out down to the original ground level about 3 feet out from trunk.  Then over time, excavated out about 6-8 feet from trunk.  It doesn't have to be done today, but soon, before the weather heats up and transpiration demand increases.  So in the next month at latest.  Start slow and gradually the mud can be moved out further from the tree.

Trees buried above bud union that need to have soil removed down to original soil level.

[From the UC Master Gardener Program Statewide Blog]

A flooded backyard in Elk Grove, Calif, following exceptional amounts of rain in January. Taking steps to improve drainage and reduce water damage following flooding is crucial to the health of your plants and lawn. Photo credit: Era Schroepfer, used with permission.

Proper irrigation and drainage are critically important for the health of plants and trees. But what happens when Mother Nature throws an atmospheric river curveball, and your yard or garden is now under water from heavy rains or floods?

Good garden soil contains a network of pore spaces filled with water and air. Both are necessary for healthy roots and beneficial soil-dwelling organisms. When the pore spaces fill with water, air is no longer available to the root system, and the roots become susceptible to root-rot organisms. Understanding the effects of flooding on plant health and caring for them after a flood event is important to saving your plants and garden.

Once the floodwaters have receded, assess the damage to your garden and begin the recovery process. There are a few things you can do to minimize the damage to your plants from flooding:

1. Remove any debris, such as mud and silt, that may have shifted and accumulated on your plants. 
   
   
2. If the soil is waterlogged, improve drainage by digging ditches or furrows to redirect water away from plants. 
   
   
3. Check the soil for compaction and loosen it up with a garden fork. This will help to improve drainage and make it easier for water and nutrients to reach the roots of your plants. 
   
   
4. Wait until the soil dries out before working with it in order to reduce additional compaction. Avoid walking on waterlogged soil to prevent compaction and further root damage. Stay off a boggy lawn!
   
   
5. Inspect your plants for damage to the roots, leaves, and stems. Remove any damaged parts, and prune your plants back to healthy growth if necessary.
   
   
6. Remove contaminated material. Consider that any garden produce touched by floodwater may be contaminated and discard it. While the risk of contamination is low in residential areas, runoff from septic systems, pastures, or industrial areas can carry potentially harmful microbes and chemicals.
   
   
7. Monitor your plants closely for signs of stress, such as wilting or discoloration, and address any issues that arise as soon as possible.
   
   
8. Once dry, start to water your plants gently and gradually to help them acclimate to the new soil conditions. 

Connect with us! 

Recovering from a flood can be a difficult and time-consuming process, but with proper care and attention, your garden can recover and thrive. The UC Master Gardener Program is available to help! For gardening questions and local county resources, click here to Find a Program. You will be redirected to your local county website and contact information. 

Source: Flood: Plant Stress in Extreme Wet Conditions, https://marinmg.ucanr.edu/PROBLEMS/EXTREME_CONDITIONS/Flood/

Avoid walking on waterlogged soil to prevent compaction and further root damage. Stay off a boggy lawn! Photo credit: Erica Schroepfer, used with permission.

After several multi-year droughts over the last decade, I find myself puzzled by the moisture that's been falling from the sky here in Auburn this winter! I find myself even more puzzled by the saturated state of our foothill soils! Over the last several weeks, as I've driven Highway 49 between Auburn and Sonora, I've seen creeks running that haven't flowed in the last several years. After years of worrying about drought and wildfire, I find myself worrying this year about flooding.

In the last two weeks here in Auburn, I've measured more than 11 inches of rain. Our total for the water year (which started October 1) is more than 23 inches - more than we measured in all of 2020-21! And if the 7-day forecast is correct, we'll measure another 6 inches by next Tuesday. In other words, we're wet and going to get wetter!

Over the last two fire seasons, we've implemented a Livestock Pass Program here in our foothill counties (Nevada, Placer, and Yuba). Rather than focusing on evacuating livestock during an active emergency, the program provides training for ranchers to allow access within evacuation zones to care for livestock. While our focus has been on wildfire evacuations, recent flooding in rural parts of Sacramento County reminds me that there may be other types of disasters (natural and otherwise) for which we need to prepare.

During fire season, I carry 5 gallons of water and a fire tool in my pick-up. While I've never actually had to use either of these, I feel better knowing that I have at least some capacity to deal with an emergency. But it's been so long since we've had this kind of winter weather, I've had to think about what I need to carry in my truck this winter.

Unlike the Sacramento Valley (where all of this water we're seeing in the foothills eventually winds up), our region typically doesn't face total inundation. That said, rainfall like we've experienced in the last several weeks makes small creeks run and bigger creeks rage - sometimes across the roads I use to access our sheep before and after work. And the combination of saturated soils, high winds, and drought-weakened trees can cause problems, as well - with fencelines and power lines alike. Since we use portable electro-net fencing, I spend a few extra minutes each day walking the fence to make sure nothing has blown over. This week, with more wind and rain, I put my chainsaw and shovel in my truck.

Someone asked me this week how my sheep chores change in weather like this. I jokingly answered that not much changes, other than the fact that I get wet while doing them! But that's not entirely true - I find that my grazing planning does change when we're in a wet pattern like this. Since we won't be lambing for another 6-7 weeks, I'm not too worried about making sure there's shelter from wind and rain - the ewes are pretty hardy at this stage. But I do adhere to the old shepherd's adage that the best shelter for sheep is a belly full of grass. Just like I try to keep my gas tank at least half full during fire season, I try to make sure the sheep always have enough feed to be ok if I can't get to the paddock for a day or two.

And with the rain we've had so far, we seem to have enough grass! For now, anyway!