When to Prune & Fertilize Plum Trees in Southern California

Winter Pruning

• Pruning plum trees is primarily a winter activity, with best results achieved when the trees are dormant. When sap is flowing, there is much greater stress to the tree. Trees are not fully dormant until well after leaf drop and several weeks before the buds begin to swell at the beginning of the next season. In the lengthy growing season of southern California, plums don't go fully dormant until December and begin to bloom again in February, so the only window for winter pruning is in January.

Spring and Summer Fertilization

• In southern California, fertilization can begin once the trees have started to flower in February. A good rule of thumb is to use two cups of all-purpose fertilizer for every inch of trunk diameter during the first flush of growth in spring. Fertilize at this rate once a month, then cut the rate in half for the summer. Immediately follow fertilizing by thoroughly watering the root zone during the rainless growing season of southern California.

Late Summer Pruning

• There are a couple reasons to prune plums during the growing season in southern California. Plums suffer from a host of fungal diseases that are exacerbated under moist conditions. Because pruning opens up the flesh of the tree, it can be an avenue for pathogens to invade. In southern California, growing conditions can be quite moist in winter but are always extremely dry in summer, making this the best time to prune diseased trees. Also, winter pruning has a stimulating effect on the growth of plums and is not useful when trying to control the height of the tree. Summer pruning is much more effective for this purpose and should be performed once all the fruit has been harvested. Plums begin to ripen in early June in southern California and the late varieties are harvested in early September. By the end of summer, it is safe to prune the trees. The help control the spread of plant diseasel, all pruning tools should be sterilized by dipping them in rubbing alcohol before pruning plums, as well as between pruning each individual plum tree.

Fall Composting and Mulching

• Concentrated fertilizer is not necessary during fall in southern California. However, growers may wish to replenish organic matter and micro-nutrients with an application of compost. The nutrients from the compost will slowly work their way into the soil and be available for the tree during the next growing season. Spreading 2 or 3 inches of compost over the root zone helps to maintain soil fertility in the long run. An additional layer of mulch on top of the compost also provides a slow release of nutrients and helps to protect the soil from the heavy winter rains that sometimes hit southern California.