One reason I enjoy my work is being outdoors a lot. Even when the weather is wet, as it was in early March, I can get out there and get a lot done while enjoying the sensations of a green and rainy winter.
Though many a client prefers to postpone a visit if the weather is wet, some enjoy a rainy day as much as I do. A couple of weeks ago, I toured a property for about an hour and a half with the owner as a light rain gradually increased to a downpour. We checked out a number of tree issues without a problem, except that my formerly waterproof boots soaked through. (And note-taking is tricky. I’m enjoying my new “Rite in the Rain” notepad and I realized the Voice Memo feature on my smartphone serves well as a notes recorder. And an inverted paper coffee cup serves well as a rain cover for the phone!)
On another rainy day, last week, I had the opportunity to assess the condition of a row of mature olive trees located between a driveway and a vineyard. Some were showing poor vigor and dieback of outer branch tips.
There could be a number of causes for this including various diseases and pests, but the underlying likely cause was staring me in the face on that rainy day: standing water in the root zones.
Here is another one: In a parking courtyard that has about 20 London plane trees, most are growing well, but one stands out as the weakest of the bunch. On a recent rainy day, it was the only one that had standing water puddled at the base of the trunk, a hint that drainage is poor at that spot, or the soil is compacted, or both.
Days like those provide opportunities to observe and learn a lot.
Drainage is a huge issue for trees. Rain-drenched or irrigated tree root zones saturated for too long deprive roots of oxygen essential for respiration. Yes, roots absorb oxygen and release carbon dioxide.
Saturation fills the soil pores with water, displacing gases. Occasional, temporary saturation can be good for tree health. It displaces built up CO2 released by roots and soil organisms. Subsequent drainage allows fresh, oxygen rich air to diffuse again into the soil, allowing roots to “breathe.”
Chronic root zone saturation favors disease, like walking around too long in wet boots and socks. For human feet, the moisture favors certain fungi. (I looked it up and found the cause is typically one of two species of Trichophyton). For trees, the usual disease organisms are various species of Phytophthora — water molds technically classified as Oomycetes — and species of Armillaria, commonly known as oak root fungus.
Compaction coupled with poor drainage shapes the root structure of trees, with major implications for structural stability. Too-dense soil and lack of oxygen prevents root growth, resulting in predominantly shallow roots and poor anchorage.
Mature California native oaks often fall victim to drainage problems. I saw a sad example of this about three winters ago. A big valley oak had a stone tree well built around it to hold fill soil away from the trunk after the root zone grade had been raised about 3 feet for the new landscape many years ago. After a period of saturating rains, the tree failed at the roots. The upturned base revealed dead and decayed roots and a pool of standing water, trapped under the tree by the grade change.
It is critical to recognize that valley oaks spend decades, even centuries, growing roots where conditions are favorable. They are not able to rearrange their massive root systems to deal with big changes. They can tolerate flooding while dormant in rainy California winters, as long as the water recedes in spring. They thrive on stream banks, not in the water.