A tractor-mounted string blossom thinner imported from Germany has garnered a lot of interest from growers in recent years, and it’s no wonder. Some growers have reported terrific results. Paul Van Konynenburg, who farms cling peaches west of Modesto, CA, said at a trial last year that he bought one of the $16,000 units, a Darwin PT 250, prior to the 2010 season, and it paid for itself in that one crop year.
Researchers have reported the thinner not only saves growers money on thinning costs by reducing the amount of hand labor, it boosts growers’ gross income because of increased fruit size due to thinning earlier in the season. The sooner growers thin, the less inter-fruit competition they have, and the remaining fruit have a longer time to size up.
However, it’s important to note that many of these impressive results came in a warm, short growing season and generally heavy sets in the test orchards — ideal for a blossom thinner. In addition, the trees were trained to a “V,” not the more traditional vase training system. When the trees are trained to a “V,” the Darwin’s 24-inch rubber cords, which are attached in columns along a 10-foot boom, can easily thin what is essentially a wall of flowers. However, many orchards are trained in the traditional vase shape, and that’s an important consideration, notes Janine Hasey, a University of California Cooperative Extension farm advisor who recently presented her findings from the 2010 and 2011 crops in the Sacramento Valley.
In fact, most peach growers in the Sacramento Valley can’t currently use the Darwin unit because they employ conventional vase-shaped systems, says Hasey. However, she notes the vase system can be modified into what is called a renewal pruned orchard, though it does take several years to make the transition. Renewal pruned vase-shaped trees also have the advantage of not having any secondary scaffolds. Secondary and tertiary limbs promote shading out anyway. The conventional lower hanger ends up getting shaded out, and fruit quality suffers.
“If you really want to mechanize, the best training system is the quad V. If you have vase-shaped trees, you have to do renewal pruning. This goes for mechanical harvesting too, which we do up here to a degree, because the fruit can drop without hitting more branches and bruising,” says Hasey. “If you’re looking at labor-reducing strategies, you’re looking at more mechanization, which means V training. But at least with renewal pruning, you can still have a vase shape and get a good response.”
It’s also important to note that in addition to the training system, the variety selection will have a great impact on results. Because it’s important to thin the early season varieties as early as possible to reduce competition and give the remaining fruit more time to size up, the thinner will work best on them. Growers won’t see such potentially dramatic increases in yields in the varieties harvested late in the season. But if your training system is appropriate for mechanical thinning, and you have an extra early variety such as Carson, Hasey says a grower should definitely consider a unit like the Darwin. “Varieties that are extra late or don’t need a lot of thinning, like the often light-setting variety Starn, are not amenable,” she says.
Weather A Factor
Another factor for growers to consider is that the years she did trials, 2010 and 2011, were cool with a long season to size fruit — not optimum years to test.
Growers predict harvest timing based on the weather 30 days after bloom. If it’s cool, growers can keep more fruit on the tree. If there is warm weather 30 days after bloom, an earlier harvest is predicted, which means there is less time to size fruit, so growers have to do earlier thinning. “You’d expect more response with this system in such a warm year, because whenever you can go in earlier you’ll get better response,” she says. “These past two years the fruit didn’t respond as much, which is to be expected.”
Overall, says Hasey, the best strategy is that if a grower is going to use the Darwin, he also has to use common sense. Simply aim for the same number of fruit per tree as you would leave on if you were hand thinning only, and try for a lower overall thinning cost. Try for the same yield; you may indeed see a greater yield. “The first year we over-thinned because crews didn’t adjust for fewer fruit on the tree,” she says. “Yet even though we over-thinned, we had higher yields in two of the five orchards tested, and with lower thinning costs, we saw a net profit over the hand-thin only because we came out with heavier fruit.”
The mechanically thinned trees didn’t require much follow-up hand fruit thinning. If you do accurate fruit counts, you should do pretty well with the Darwin if the other factors, such as training system and correct variety, are in place and the weather is favorable. “It’s mainly a way to reduce thinning costs,” she concludes. “Count your fruit, know what your trees can size, and if it’s a short season to harvest, thin harder. In a longer year to harvest, you have more time to size fruit.”