There are about 2,500 species in the palm family, according to specialist Don Hodel, author of The Biology and Management of Landscape Palms. In my experience, only about 11 are viable here in the Napa Valley area climate zones and 3 comprise the great majority of palms found in local landscapes:
- Canary Island date (Phoenix canariensis)
- Mexican fan (Washingtonia robusta)
- Windmill (Trachycarpus fortunei)
Others found here less commonly include California fan, Mediterranean fan, Queen, Guadalupe green and blue species, date, pindo and Chilean wine palm.
We have well over 200 species of temperate climate-adapted broadleaf and conifer trees growing here but palms are limited to very few that are relatively cold hardy. Of those, the Canary Island date palm stands out for its impressive, massive presence. They have long been placed as a sort of landmark announcing to visitors: “Welcome! You have arrived!”
Large specimens are readily transplantable, where budgets allow. They typically survive the process if handled well and go on to provide a long-term contribution to the landscape.
Ironically, the healthiest local specimens I know of are growing wild in the riparian lowlands just south of Imola Avenue.
They do have their particular needs and problems. And they have quirky ways of expressing them. Quoting Hodel: “…a palm trunk is an indelible record of a palm’s past cultural history because it lacks the ability for secondary growth and, thus, cannot cover wounds or increase the diameter of a constricted part of a trunk produced during a time of stress”.
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A descriptive term for trunk constriction is “penciling”. During a time of drought or other stress, growth is reduced and the trunk diameter decreases; it tapers like a sharpened pencil. It increases during periods of good vigor. Over many years, the trunk develops undulations of diameter along its length, a visual record of its life history.
I saw a good example on a recent visit. The owner was concerned because of excessive yellowing of older fronds. That can be a result of various causes. A common one is mineral deficiencies, Magnesium in particular. But the root zone was not well irrigated, and the upper trunk exhibited the tell-tale penciling. The palm survived its transplanting over 10 years ago, but irrigation was inadequate during the recent years of drought.
The yellowed fronds cannot recover normal green color. But with good care, the new fronds that will appear over the coming years will be of normal color and the trunk diameter will increase where it rises above the penciling.
While we are on the subject of this species, a few words of caution. Canary Island date palms are susceptible to an always fatal fungal disease called Fusarium wilt. It has been confirmed as present in Napa Valley. Infected palms show the death of leaflets on one side of the frond midrib. Eventually, the palm dies. The disease is easily spread by chainsaws and other pruning tools. Where an infected palm has been removed, a replacement palm is just about certain to become infected.
Palm owners and tree services should be aware and get up to date on the disease prevention protocol:
- Thoroughly clean hand saws and other pruning tools after pruning by vigorously brushing off sawdust and debris.
- Disinfect tools for 10 minutes in a 1:3 pine oil-to-water solution or full-strength bleach or disinfect saw blades for ten seconds per side with a butane torch.
- Chainsaws are impossible to disinfect, so do not use them on palms if there is any chance they have been used on Fusarium infected palms.