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Agri-Fos is phosphate compound formulated to stimulate growth in trees. Research has shown that about 80% of trees respond well to treatment, meaning that their defenses are stronger. Applications should be made when the tree is actively transpiring. Avoid treating trees during very hot or very cold weather.  Also, avoid applying the compound when deciduous oaks have lost their leaves or when new leaves are emerging in the spring.

Currently in Northern California two applications in the first year are recommended, one Fall treatment in late October through early December or a Spring treatment, March through May, and a second treatment approximately six months later.  Subsequent treatments should be made once every 12 to 24 months. Expert opinions on how often to spray are evolving.  Please check additional resources for the most current recommendations.

An alternative to spraying particularly well suited for large oaks, (greater than 20”) or oaks covered in moss is direct injection.  Injection uses diluted Agri-Fos (no-PentraBark) and requires much less product as compared to spraying. If you do contact an arborist, you may want to discuss injection versus spraying.

Preventative treatment, before infection has occurred, has been found to be more effective than curative treatments.  At least 4 weeks are necessary for the applied chemical to take full effect.

Agri-Fos should be mixed per the manufacturer’s directions provided below. Many homeowners elect to do this work themselves. Caution: Agri-Fos is relatively benign, but the Pentra-Bark compound it is typically mixed with can irritate the lungs and the skin as well as cause nausea and head aches. Avoid "over spray" and heed the manufacturers instructions on wearing protective equipment.

For the non-do-it-yourselfer, we've heard recommendations for the following spraying contractors.

Specifications For the Spraying of Agri-Fos

Directions for Mixing Agri-Fos and Pentra-bark: Mix Agri-Fos® with Pentra-Bark® and water in the proportion specified on the label, which yields one gallon of spray: 62.4 oz (1.9L) Agri-Fos® + 62.4 oz. (1.9 L) water + 3.2 oz (95 ml) Pentra-Bark®. Mix in the following order: water, then Pentra-Bark®, then Agri-Fos®, so that the Pentra-Bark® will dissolve properly in the water and will not precipitate out at the bottom of the sprayer. Gently shake and pour into sprayer.

Directions for Spraying: Apply uniformly up 6 - 9 feet of trunk circumference. Spray from top down to ground level from either first branch or from as high as possible without exposing applicator to drift. Spray to just prior to runoff. To avoid waste of Agri-Fos, portions of trunks with moss that is so heavy as to interfere with spray reaching the tree's bark may be brushed to remove excessive moss, or if a relatively small area, may be sprayed around. The solution will likely discolor or kill moss and lichen.  Can be used as a preventative or curative application. Wear protective gear: gloves, mask, long sleeves and pants.  For more info, see “Protecting Your Oak Trees from Sudden Oak Death” at