Garden Tolerance

Planting natives

If you’ve been reading this column, you’ve read a lot this spring about planting California native plants. They are among the best choices for the areas in your garden which you plan to leave unwatered or less watered this summer. Not only are they wonderfully adapted to our long dry summers, they also support native wildlife and soil microorganisms. When they are grouped together, they can create a permanent, sustainable community of garden plants. With good design choices, they can also be stunningly beautiful year-round.

Fremontodendron  (California flannel bush).

Fremontodendron (California flannel bush).

So why did we cut down our Fremontodendron?

“Wait! You did what? You cut down that big, beautiful California flannel bush with its loads of huge butter-yellow flowers and it's handsome arching branches? What the heck is going on over there?”

Well, as a matter of fact, we had a very good reason for do so. If you had looked at the base of the trunks of this beautiful and beloved plant, you would have seen fungus, lichen, and rot of various kinds. Once we cut it down, you could see inside the stumps how the fungus had penetrated and was rotting the heartwood of each trunk, which had turned brown.

Garden tolerance

Over the past years, we have been slowly developing an orchard on our steep hillside. We have been terracing, building pathways in the sandy, eroding soil, and adding more varieties of fruit trees. At last count, we now are representing 57 fruit trees. This orchard is partly an experiment, to see which fruit varieties do best in the sandy, foggy, windswept Sunset, without very many cold winter hours. It is also part of our small food justice effort, in which we provide fruits and vegetables to a homeless youth shelter each week.

Orchard trees require regular fertilization and watering. California native plants hate regular fertilization and watering. It makes them sick in many different ways. An established native plant (like our Fremontodendron) which is suddenly exposed to fertilizer and summer water will grow enormous and bloom prolifically for several years, and then die. Unfortunately for our Fremontodendron, it was located below the orchard, so it was constantly exposed to orchard water seeping downhill into its root zone.

The North Orchard at GFE in 2007, looking downhill towards some native plants.

The North Orchard at GFE in 2007, looking downhill towards some native plants.

Some native plants can be mixed in with other plants requiring summer water, but most can not. This is called the “garden tolerance” of the native plant. Most native plants have poor garden tolerance, which just means they will not thrive if exposed to normal garden conditions like rich soil, fertilization, and summer irrigation. They may even die.

So does that mean that all the native plants at GFE are dying? Not at all. Unfortunately for the Fremontodendron, it was planted below the orchard. However our thriving native backyard, wild hillside, and demonstration path are located either above or between the orchard areas, without fruit trees above them. Since soil moisture does not flow up, these native areas are safe and sound.

Hydrozoning

Hydrozoning is the practice of grouping plants together based on their water needs. This will be extraordinarily important in the coming years for San Francisco gardeners. Most of our gardens need to be designed or redesigned with water-wise plants, which need little or no summer irrigation. Small areas of the garden can be planned for veggies, or for a few garden favorites like roses or rhododendrons which need moist soil, but most of the garden should not need much summer water. This is just common sense as we move into another year of drought.

Not only California natives, but also water-wise plants from Mediterranean climates around the world, are very vulnerable to disease if exposed to too much summer water. So they should not be mixed with garden plants which need irrigation. They need their own area where the soil can be allowed to dry out in the summer.

Furthermore, the dry zones, or rain gardens, or xeriscapes in our sometimes steep or sloped gardens must be located above any watered areas, or they will face an invisible enemy... water seeping downhill from watered areas, either in your own or neighboring gardens. Otherwise you may end up like us, having to cut down your best and favorite plants.