Ornamental fruit trees such as crabapples and the Bradford pear produce large crops of inedible fruit. Older standard apples may grow beyond the ability of homeowners to tend properly and bear many unwanted small wormy apples. Fruit thinning sprays reduce the quantity of fruit that reaches maturity and lessens cleanup problems. Not all fruit trees respond to spray thinning. Some thinning chemicals poison bees and other beneficial insects.
Mark the date of the tree's full bloom on a calendar. Timing the spray correctly determines whether the treatment works. Sevin insecticide thins apple and pear fruit if applied no sooner than five days after full bloom and not later than 15 days after full bloom. Spraying with NAA (naphthaleneacotic acid) works if done between seven and 21 days after full flowering.
Mix Sevin insecticide according to the manufacturer's directions. Sevin alone thins apple and pear fruit but the level of success depends partly on the variety of tree. Commercial growers often prefer NAA but nurseries seldom supply this chemical for home use. If using NAA measure one gram with the scale and add the chemical to one gallon of water. Add 2 1/2 tbsp of spray oil. Mix thoroughly.
Put on protective clothing as specified by the manufacturer of the chemical in the thinner. Expect the mist to drift even in calm air. Avoid unnecessary exposure even to chemicals labeled safe.
Spray the fruit trees on a warm day--above 75 degrees F--when the air is still. A forecast of calm and dry conditions increases the effectiveness of the spray since the chemical remains on the tree longer. Wet weather after application reduces the impact of the thinner.
Fill the backpack sprayer with the appropriate solution and pump it to full pressure. Spray all blossom and fruit clusters thoroughly. All blossom and fruit surfaces must be wet with the thinner for the treatment to work properly. Fruit drop should begin within a few days.