Elinor Teague on Gardening: Trees can’t wait for El Niño rains

The autumn leaves began falling at least two weeks earlier than normal this year.

Many deciduous trees are so drought-stressed that they are entering into dormancy several weeks ahead of schedule and ahead of the fall/winter rains. We need to deep-soak root systems now, just as the trees enter premature dormancy, when non-dormant roots and interior water flow systems can still take up moisture in sufficient amounts to ensure the trees’ survival through the winter. We can’t wait for the predicted El Niño heavy rains (or assume that they’ll arrive) to deep irrigate our mature landscape trees.

Place soaker hoses, drip emitters, bubblers or oscillating sprinklers at the edge of the drip canopy where the absorbent root tips lie and allow water to slowly penetrate at least 12 to 18 inches into the soil at the edge of the canopy over a period of several hours on mandated watering days. Continue to deep-irrigate landscape trees weekly until the first frost arrives or until we receive significant rainfall. The first rains will quickly run off crusted soil. It may take several storms before rainfall wets more than the top few inches of soil.

When the rains do arrive, the splashing drops will spread fungal spores that have lain dormant on fallen leaves and twigs and on the soil surface for months or even years. You may have noticed that during these last four drought years we have seen less serious outbreaks of anthracnose, peach leaf curl, black spot and shot hole disease. Dry weather prevented the spread of the spores. If we receive higher than average amounts of rain this winter and spring, we will see increased problems with fungal diseases next spring when wet weather, higher humidity and mild temperatures provide ideal conditions for spores to spread and proliferate.

Pick up all fallen leaves and tree litter regularly, paying special attention to areas underneath trees which have shown signs of fungal problems. Root injection of fungicides to treat for anthracnose in sycamores, elms and oaks is not recommended by UC Davis Cooperative Extension. Spraying large trees with fungicides is expensive, not always effective and timing is tricky. Anthracnose seldom kills trees; cleaning up leaves and tree litter that harbor anthracnose spores will minimize fungal spread. It is now recommended that one spraying of liquid copper to treat dormant peach and nectarine trees for peach leaf curl and shot hole disease (also apricot) be applied in early spring at bud break. Since timing spray applications between spring rain showers is not easy, some gardeners still spray in autumn and also spray the fungicide on the soil underneath infected trees at the same time.

Pruning of deciduous trees begins at leaf fall in autumn. So many landscape trees have suffered severe limb dieback during the drought. Many trees are carrying dead, hazardous branches and their structures have become unbalanced due to limb loss. Poor pruning will further stress or even kill the trees. Hiring licensed tree care companies that have certified arborists on staff is well worth the money.

This story was originally published November 11, 2015 at 6:30 AM with the headline "Elinor Teague on Gardening: Trees can’t wait for El Niño rains."