I love oak trees. They are stately, enormous trees, very well behaved with deep roots and excellent branch structure. Long-lived: Oaks are wonderful trees to plant for posterity, to honor someone, to provide a grand canopy across a large lawn or public area. The answer is: Leaves that don’t drop, and leaves that do.
So it always pains me to have to recommend against them for homeowners. The problem isn’t that they don’t grow here; most species do just fine. One of the premier collections of the genus Quercus, the Shields Oak Grove, fills the west end of the UC Davis Arboretum. If you have sufficient room and can deal with the issues below, please consider making room for an oak tree. A grand, spreading oak can be a great asset to a neighborhood or park.
Leaves that don’t drop when they should.
With many deciduous oaks, the problem is marcescence (pronounced mar-sess-ense). This is the condition in which a plant doesn’t shed its leaves when it should. Unfortunately, a lot of eastern species of oaks are marcescent. They aren’t evergreen: they’re technically deciduous because the leaves do, in fact, die at the end of the growing season. They just don’t fall off.
This all came up recently via an email inquiry:
“I have a 10-year-old pin oak (Quercus palustris) … that I want to get rid of because the leaves do not drop in the fall. I have purchased a scarlet oak (Quercus coccinea) and am finding conflicting statements … about whether the leaves of the scarlet oak drop in the fall or wait until the tree leafs out in the spring. … I would hate to plant the scarlet oak only to find the same problem.”
While pin oak is probably the worst species in this regard, other oaks native to the Eastern United States are also marcescent. What that means is they turn color in the fall, often a pretty shade of yellow or red, and then the leaves turn brown. And then they hang on, still brown, until winter storms blow them off in January or February. Or, in the case of pin oak, they finally drop as the new leaves come in the spring.
A little science: The first step in leaf drop for a normal deciduous tree is development of an abscission layer of cells that forms at the base of the leaf in early fall. These cells create a separation zone where the leaf detaches.
I would be reluctant to recommend any Eastern U.S. species as having reliable leaf drop in the fall; formation of the abscission layer is not consistent. In my opinion, you will have the same problem with Q. coccinea that you have with Q. palustris, though to a slightly lesser degree. A less effective separation layer forms in scarlet oak in January, and not until almost March in pin oaks.
Red oak (Q. rubra) and turkey oak (Q. shumardi) follow the same pattern as scarlet oak. The trees are healthy, providing lovely shade and wildlife habitat, and sturdy enough for climbing. They just have a lot of brown leaves hanging on when they should be bare.
West Coast species of deciduous oaks don’t have this problem, including our stately valley oak (Q. lobata). The leaves drop clean when they’re supposed to, though the fall color isn’t notable. Valley oak is slower growing, though eventually attaining great size. More neighborhoods and greenbelts should include valley oaks. But a homeowner looking for shade may wish to choose a faster-growing tree.
Leaves that drop all the time.
The complaint about evergreen oaks has to do with their litter. These species hold leaves year-round, but there is a steady shedding of leaves and a heavy drop of older leaves in the springtime. The amount of leaves on the ground can be considerable, making it a little hard to maintain lawns and garden beds beneath them.
The most common evergreen oak in our area is the non-native cork oak, Quercus suber. These were widely planted in town and on campus in the 1940s, so there are some good-sized specimens. Cork oak is easily recognized by the soft, spongy bark, used for making wine corks. It’s actually a native to Portugal and Spain, where the rainfall pattern is much like ours, so they are well adapted to the valley. As with our native plants, cork oak doesn’t want much summer water. It is an excellent tree in a low-water landscape.
Oak leaves are thick and decompose slowly, and the amount from a large tree can make deep mulch. But they are not harmful to the garden at all, breaking down to provide great organic matter and enriching the soil. Which brings us to our next topic.
Leave the leaves!
Recent discussions about possible changes to our waste removal and brush pickup program in Davis reminded me that many of you may be wasting a great resource for your garden: leaves and lawn clippings.
Leaves are entirely beneficial to the soil and the garden. You can rake them up, then spread them out around shrubs, trees, fruit trees, or in vegetable garden beds that you’re not actively planting during the winter. The earthworms will happily feed on them, help to break them down, and they’ll enrich your soil.
But you don’t even have to rake them up in many cases. If you have a mulching mower, and conditions aren’t too wet, you can simply mow right over the leaves. It may take a couple of passes, but your mower should be able to pulverize them into pieces. They can fall right back into the grass, turning into mulch. I have seen studies where leaf layers up to 12 inches deep or more were simply mowed repeatedly and allowed to filter into the turf and return to the soil.
A landscape gardener in Southern California sent me some pictures of a technique he’s adopted. Landfills there are getting more expensive and particular about what they will accept. So he spreads the leaves out on the driveway, then takes several passes over them with a mower to reduce them to leaf mulch. Then he simply spreads them out around the shrubs.
Lawn clippings can be very beneficial, but it’s important that they be pesticide-free if you’re putting them elsewhere in the garden. If you’ve been using weed killer on the lawn, there is risk of harming non-grass plants when used as mulch, and you definitely don’t want to use them in the vegetable garden. Lawn clippings from treated lawns need to be disposed of carefully if they’re taken off site. But if they are just allowed to filter back into the lawn, they won’t do any harm.
If you’re fertilizing your lawn regularly, the clippings have a lot of nitrogen in them. They can be put into a working compost pile, or spread out around shrubs and trees or in your vegetable garden. But as with leaves, the simplest is to just let them lie where they fall if the appearance isn’t an issue. They will decompose and become organic matter for the soil.
So bottom line: the only things you really need to put out in the street for pickup are branches, weeds and prunings. We could all reduce the amount of yard waste pickup by thinking of it as a resource, not as waste. Let the leaves and clippings go back to the soil!
Things in leaves?
A new pest has made its way into Northern California and I have now seen some samples from around the Sacramento area.
Citrus leafminer is the larva of a small moth, and it is literally inside the leaf. Leafminers are oviposited beneath the epidermal layer and spend their time tunneling through the leaf. The pattern can be very interesting, and there is some loss of chlorophyll where the larva is active.
But the impact on the tree, even in the case of heavy infestation, is minor. The adult can only lay its eggs on softer leaves, so the problem is noticeable on new growth in the late summer. In the last stage of development the larva emerges from the leaf tunnel, moves to the edge of the leaf and rolls it inward around itself. So the leaves get some distortion and curling.
It is harmless, and it’s neither practical nor necessary to spray. There is no damage to the fruit.
Another similar pest, the citrus peelminer, also has worked its way up into Northern California. As the name implies, the damage is on the outer part of the citrus peel. Again: harmless, although commercial growers will be treating their trees because of the cosmetic damage to the appearance of the fruit.
— Don Shor and his family have owned the Redwood Barn Nursery since 1981. He can be reached at firstname.lastname@example.org. Archived articles are available on The Enterprise website, and they are always available (all the way back to 1999) on the business website, www.redwoodbarn.com