Hello again my garden friends.

Here in the Westside, the majority of our trees are deciduous; this means they drop their foliage in autumn.

After the leaves shed, the roots remain active until the soil becomes too cold. As a result, the tree continues to benefit from potassium, well after the leaves drop. It is very important to use high-quality organic fertilizer; it should contain high quantities of potassium. Potassium is vital for root development.

Also, the tree fertilizer needs to contain a certain level of slow temperature release. The polymer-coated particles will hold back nitrogen until the soil cools. The nitrogen will remain in the soil throughout the winter. As the soil warms in Spring, the fertilizer will be readily available to help the tree leaf out.

A common misperception is that trees do not need fertilizer in autumn. This is not the case. Trees can benefit greatly from the correct fertilizer application within the mid to late autumn. The key, then, is selecting the correct fertilizer. Most general tree fertilizers contain large amounts of quick-releasing nitrogen. This promotes fast, weak growth and foliage.

Any other questions regarding fertilization, eradicating insects and fungal issues, please feel free to contact me.

Mark Koehler

Mark Koehler of Los Banos is an arborist and master gardener, who has degrees in Landscape Architecture and Landscape Horticulture from UC Berkeley and Northeastern University.