by Charlene Burgi
"It must be true: I read it on the internet" is a joke we hear often. Gardening blogs and websites can be a great source of information on irrigation, landscape maintenance, soil preparation, mulching, and dealing with rodents, deer and insect pests. But as we enrich our knowledge online, we also need to seek out local knowledge and consider the unique growing conditions in our own gardens.
This week a piece of information crossed my path from a well-thought-of website that recommended irrigating vegetables with one inch of water per week. Needless to say, I was taken aback by this advice. The question that came to mind was: What is the evapotranspiration (ET) in that area? Surely the writer is not saying all veggies worldwide need one inch of water? The problem is, if readers don’t know how to adapt that advice for the ET where they live, they could be overwatering or underwatering their plants. But if it’s on the internet it must be true, right?
Put simply, ET is the measurement of water that is lost from the soil through evaporation and from plants through transpiration. (Humans perspire, plants transpire.) When we irrigate, we replace the water lost through these natural processes. If we do not provide enough water to compensate for the current ET, our plants could experience wilting or burned leaf tips. Even if adequate water is available, a plant may wilt or burn in extreme heat if it is unable to transpire fast enough to compensate for the heat. On the other hand, too much water can contribute to disease, soil compaction, weed growth and root rot. It can also cause fertilizers and naturally occurring nutrients to dissipate or wash away. Finding that “sweet spot” with just the right amount of water—not too little, not too much—helps keep plants happy and healthy.
| Rain gauge
What is the formula for applying just the correct amount of water? Check MMWD’s Weekly Watering Schedule
for the runtimes for your climate zone within Marin and adjust your controller accordingly. (Smart controllers will do this automatically.) The information found on the site is based on the current ET for our area, so your plants get just want they need. If you water by hand, invest in a rain gauge that will trap the overhead water for a reading; compare that to what they need and adjust accordingly. Don’t forget to add mulch to reduce the evaporation from the soil and help keep the root systems cool.
Trust me, blanket statements about how much to water are not the best way to achieve a healthy garden. And you did read that on the internet!