Cherry researchers used this cherry color chart in the harvest timing project. For chart purchase information, email Todd Einhorn at: todd.einhorn@oregonstate.edu.

Cherry researchers used this cherry color chart in the harvest timing project. For chart purchase information, email Todd Einhorn at: todd.einhorn@oregonstate.edu.

Photo courtesy of Lynn Long, Oregon State University

Determining when to harvest sweet cherries is a balancing act between weather, fruit quality, and market demands. But for most varieties grown in the Pacific Northwest, growers should pick in the 4, 5, or 6 range of the French Ctifl color chart, says an Oregon State University Extension educator.

“Four would be the earliest that cherries should be harvested,” said Lynn Long, OSU ­Extension educator for Wasco County. “You want to avoid 2 and 3, but 4, 5, and 6 are all acceptable. You don’t want to give up fruit size—you really can’t afford it—unless you have a good reason for it, like hot weather coming.”

The numbers Long refers to are found on a cherry color chart guide developed by the French ­Technical Center for Interprofessionals of Fruits and Vegetables, known as Ctifl. Cherry researchers in Washington, Oregon, and Michigan use the Ctifl chart to standardize fruit skin color evaluations; the chart was used in a collaborative research project between the Washington and Oregon scientists.

The project studied the optimum harvest timing for ten sweet cherry varieties in Washington, nine in Oregon. Researchers involved were Washington State University’s Dr. Matt Whiting and OSU’s Long and Dr. Todd Einhorn. Long shared the results and recommendations during the Cherry Institute meeting held in Yakima, Washington, in early winter.

The Northwest cherry industry has intensively studied Bings, Long said, adding that the industry knows how to grow it well. “We’ve focused all of our research on one variety and built our reputation on it, but when we get into other varieties, we sometimes lose our way.”

In recent years, Northwest orchardists have planted Lapins, Sweetheart, Skeena, Chelan, and a host of other new varieties. Some have been successful, some not so successful, he said. For the project, researchers collected data on color, size, firmness, soluble solids, acids, and postharvest storability to help determine the optimum picking time for different varieties.

Variability

One of the project discoveries was that skin color is a good, reliable indicator of ripeness, Long said. Skin color darkened with advancing harvest dates, although the rate of darkening was cultivar dependent.

In general, as fruit persist on a tree, skin color, sugars, and size all increase, while firmness and stem retention decrease. “All of the quality attributes that we tested for were significantly related to skin color,” he said, adding that flesh color also works as an indicator, but lags behind in timing compared to skin color.

When looking at skin color in relationship to fruit firmness, they found the force needed to remove stems, called stem retention, leveled off at around 600 to 800 grams of force in colors 4, 5, and 6. “Skin color was directly related to firmness,” Long said. “So, if you were picking for ­firmness, you’d want to harvest in the color range of 4 to 6.”

While the research team found skin color to be a strong indicator, they were surprised at the amount of color variability within trees. “We got tremendous ­variability in cherries from a single tree.”

For example, on the same tree, some Skeena fruit graded color 3, with soluble solids (Brix) at 20°, while fruit that was color 6 had 17° Brix. And, fruit from a Tieton tree had nearly equal amounts of fruit represented in all 6 color categories.

Why all the variability? They are working on finding answers. Light interception may play a role, but Long said they were working in small-sized trees that should have already had good light interception. Dwarfing rootstocks to control tree size, vertical training systems like central leader and spindle, and two-dimensional tree systems can all help improve light interception, he said.

Flower power

The researchers looked at timing of flowering to see if that had an impact on skin color and size. They tagged blossoms in the order of opening (first, middle, and last) and tracked blossoms as they developed into fruit.

Fruit from the early blossoms were darker at harvest, were higher in sugars, and were significantly larger than the later blooming fruit, Long said. “The early openers were a few days older, but we also think the blooms were stronger and that’s why we got larger fruit. Anything you can do to strengthen the blossom should give you larger cherries and decrease the variability.”

He noted that Michigan cherry growers have been ­following the advice of Dr. Greg Lang, Michigan State ­University, putting on heavy doses of urea in the fall to boost spur leaf development the following spring. Two applications of urea—20 pounds per acre—in a concentrated spray of 50 to 60 gallons of water are sprayed on the trees in September.

In an effort to grow strong and early blooming flowers, Long thinks there could be benefit from removing bees in midbloom. “Maybe we don’t want the bees to pollinate the later blooming blossoms. It’s something to think about.”

Using fruit quality as a harvest guide, he advises ­growers to avoid harvesting fruit that hasn’t yet reached color 4. “If you harvest before 4, fruit will be less sweet, smaller, lighter in skin color, though it will be firm, with strong stem attachment. We want firm fruit, but without giving up size or flavor. If you go to 6, you’ll have too many fruit overripe.”

In their trials, fruit picked from color numbers from 4 to 6 held up well in postharvest storage for up to 28 days.

He reminded growers not to forget consumer preferences. Consumers want fruit that’s big, dark, sweet, and firm. “Industry needs consumers to make repeat ­purchases, especially with the larger crops that we are growing. Research has shown that if a consumer has a bad eating experience, he or she will not come back to purchase cherries for six weeks.”

Growers in early districts should target color 4, he said, but added that they will give up some size and flavor. Later districts can target color 5. “But you’ve also got to watch the weather forecasts and be prepared to pick earlier than desired if high temperatures are predicted. You’ll give up size and flavor, but a sound crop is better than no crop.”

The key is finding the right balance between fruit quality, weather, and market (consumer) preferences, Long said. For most varieties, that means picking in the 4 and 5 color, but with Regina, harvest should be in the 5 to 6 range.