After a fire, plants may look dead but still be alive.
When evaluating what to do with a plant burnt by fire, the best advice is that given when a plant has been burnt by freezing temperatures: wait and see. Unless a plant is completely black, it may recover. To test for viability in trees and shrubs, remove a few dime-sized pieces of bark along the trunk or main branches. If the underlying tissue is either green or white, there is probably still some life in the tree and it’s worth waiting until spring to see if new sprouts emerge.
By the same token, a tree that shows no signs of living tissue should be removed at once, or at least chopped down to the stump, since it could topple over at any moment. Incidentally, as long as the stump is intact, there is a reasonable probability that suckers could grow out of it and, with the passage of time, grow into a shrub — or, in the case of a tree stump, all of the suckers except one, the strongest, could be cut off with the remaining sucker growing up eventually into a new tree.
When it comes to burnt trees, those with the best chance of survival are deciduous. In Los Angeles, deciduous trees — California sycamore, valley oak, white alder, ‘Modesto’ ash, and liquidambar come to mind — lose their leaves in late fall and early winter. When a tree is fully leafed out, this foliar fuel will increase the intensity of heat in the tree, and the hotter the fire, the less chance the tree has of surviving. Thus, as leaves fall, the potential for surviving a fire increases. Among deciduous trees, those with an open branching structure are best equipped to survive fire.
Even when deciduous trees are still somewhat leafy — and California sycamores do not go completely leafless until January — they will have already begun to enter dormancy, another positive where surviving a fire is concerned. When a tree is actively growing, it is burgeoning with sap that will also serve as fuel for the fire. Dormancy diminishes or completely halts sap flow.
Pine tree and other conifers have the least chance of surviving fire because of the highly flammable waxes, oils, and resins contained in their sap and bark. On the other hand, if only 10% of their needles are still vital after a fire, they may survive. On any evergreen tree, when less than 10% of the needles and less than 50% of the buds are viable, chances for recovery are slim.
There are two exceptions: ponderosa pine and sequoia. Ponderosa pine, with very thick bark and limbs which branch out high on the trunk, is the most fire resistant conifer and even when all of its needles are dead, it may yet sprout fresh shoots and return to its former self. A sequoia tree, even while dying in a fire, has roots from which new trees will grow, typically in a ring around where the mother tree stood.
A major problem with fire damaged pines is that cracks in their trunks and branches are literally open invitations to pestiferous beetles which crawl inside and lay their eggs. Such branches should be removed. Burned portions of any tree trunk, pine or otherwise, should be painted with a solution that is 50% non-enamel, water based, white latex paint and 50% water. The whitewashing will prevent sunburning on trees whose bark has been thinned or burnt off by fire. As long as there is contiguous bark on a tree trunk — and no turnk portion that is without bark all the way around — the tree may recover.
It should also be noted that trees that are planted close together are less likely to survive a fire since the heat generated will be much greater than when trees are spaced at some distance from one another.
Yet fire can also be helpful to plants, especially California natives. Ceanothus. manzanita, and Matilija poppy seeds may lie dormant for decades until a fire opens their seed coats and, with a little winter rain, they can finally germinate. Certain pine cones — such as those on bishop pine (Pinus muricata), a species well-suited to our coastal climate — are literally glued shut until fire finally opens them by melting the chemicals that keep them closed, allowing the seeds within to fall to the ground and sprout.
Aside from whitewashing burnt portions of the trunk and limbs and evaluating viability, the condition of the soil must also be addressed following a fire, since roots will be in desperate need of water. Unfortunately, fire may render the ground virtually impermeable to water penetration due to impaired drainage in the top several inches of soil. Although soil with a high quantity of organic matter is generally regarded as a gardener’s most valuable asset, it is precisely such soil that is especially susceptible to compaction as a result of fire. In such cases, take a metal rake and make rivets in the ground so that water, whether from rain or from a hose, can seep into the soil.
The preferred method of watering compacted soil is with a soaker hose. Soaker hose, which is made from recycled rubber, is of a spongy consistency. It is also highly flexible and can easily be made into a circle. The recommended watering procedure is to make a circle with the hose one foot away from the trunk and soak the ground next to the hose to the point of saturation. Then take the hose and move it further away from the trunk, beyond the initial saturation zone, making another, if larger, soaker hose circle. Continue with this procedure, making ever large concentric circles with the hose until you are one foot beyond the drip line or branch tips of the tree.
When the soil is moist at a depth of one foot, you can be confident that the roots will be safe, for now, from desiccation. Now you can put down a two inch layer of straw, not only between the trunk and the drip line, but throughout your garden or sloping terrain. The straw will act both as a water retaining mulch for the soil and as a barrier against soil erosion, in the even of heavy rain, as well. Still, between the trunk and drip line you will want to check the soil at a six inch depth once a week to make sure it is moist. Burnt soil will eventually return to its pre-fire condition, but it could take up to a year to do so.
Tip of the Week: Trees are best evaluated for removal at a height of 4 and 1/2 feet. Trees with large trunk diameters have a much better chance of making a comeback than less developed trees. Roughly speaking, a tree with a trunk diameter of less than 12 inches (at a 4 and 1/2 foot height) will require removal if its bark is severely charred and cracked. Where trees have a trunk diameter greater than 12 inches (at this height), it is likely that with selective pruning of the most damaged parts of the tree, full recovery can be expected. Where charring is less severe on smaller trunk diameter trees — that is, bark has been burnt and thinned by fire but is uncracked — selective pruning may be sufficient to allow for healing and rejuvenation.