Peach and nectarine growers in Pennsylvania have been testing a mechanical blossom-thinning machine that the state’s Ag Innovations research team imported from Germany.

Katy Lesser, Ag Innovations coordinator with Pennsylvania State Cooperative Extension in Adams County, first saw the Darwin thinning machine at the Interpoma tradeshow in Bolzano, Italy, last November.

Though she was on the lookout for platforms that might be of use to Pennsylvania orchardists, the mechanical thinner caught her eye. She took pictures to show her colleagues at Penn State and mentioned it in presentations at grower meetings. The interest was so great that the Ag Innovations team bought a machine and had it airfreighted to Pennsylvania so that they could test it during bloom this spring. Penn State Cooperative Extension and the Fruit Research and Extension Center at Biglerville provided funding.

Lesser said growers saw the potential to eliminate many hours of green-fruit thinning that is done by hand. They do not typically use chemical thinning on peaches, but would like to be able to eliminate the fruit early and reduce competition between the fruit in order to grow bigger peaches. Most of the state’s peaches are sold on the fresh market.


The Darwin machine was designed by grower Hermann Gessler, initially for thinning organic apples, and is manufactured by Fruit-Tec. Lesser said the design is fairly simple. It consists of a 10-foot-tall spindle attached to a frame that can be tilted and raised or lowered hydraulically to suit the tree architecture. Attached to the spindle are plates with two-foot-long plastic cords that fly around as the spindle rotates. The cords are somewhat like those on a weed trimmer, but stronger.

The machine is attached to the front of a tractor and driven through the orchard at between 2.5 and 3.52 miles per hour.


In initial tests, Ag Innovation team horticulturists Dr. Tara Baugher and Dr. Jim Schupp felt that the machine was eliminating too many blossoms. They were able to remove some of the plates and cords on the machine to reduce the thinning effect.

The frame can be tilted into the canopy or away from the canopy. For peaches on a perpendicular V system, it worked well angled away from the canopy.

The thinner did very little damage to the tree, though it had to be maneuvered around vigorous branches that extended into the drive row and should have been pruned off. As a result, some trees were missed by the thinner. Pruning is important in how effective the machine is.

"It does need to be a high-density fruiting wall, a very continuous plane," Lesser said. "The tree system is very, very important."

She thinks the machine is better suited to peaches than apples, because when peaches are in bloom, there are no leaves on the trees that might be inadvertently damaged by the cords.

When researchers also tested it on GoldRush apples during the tight-cluster stage, it did take off some branch tips, and the leaves looked a little chewed up, Lesser said. Sometimes, it removed complete clusters of blossoms, and other times, perhaps only three blossoms in a cluster. However, the leaf damage wasn’t serious. Streptomycin was applied afterwards to reduce potential for fireblight infection.

In the apple block, researchers found that by tilting the frame towards the canopy, they were able to thin more blossoms off the top of the trees than the bottom, which was their aim.

"You can target exactly where you’re going to thin the majority of your fruit," Lesser explained.

The cost of the machine, plus airfreight from Germany and importation expenses, amounted to just over $10,000.

This season, the Penn State horticulturists, working with agricultural economist Matt Harsh, will calculate the impact of mechanical blossom thinning on the cost of hand thinning. At harvest, they will study packouts, fruit size distribution, and the value of the fruit.

To view more pictures as well as videos of the machine in action in grower orchards, check the Web site at