As summer morphs into fall, plenty of tree fruit is harvested. In some regions, the last of the peaches, the first apples, and pears and plums all come to fruition now, ripe for harvest. But what is ripe? How do orchardists know when it’s the perfect time to pick?

The first step in that decision-making process is knowing your market. Immediate direct market sales require a fruit closer to perfectly ripe than wholesale markets. If your fruit will need to stay in storage prior to being sold at a later date, picking at the peak of ripeness is not going to work. Some fruits, such as pears, aren’t meant to be picked perfectly ripe, whether they are destined for long-term storage or not.

Even when the time is right for optimal picking, the appearance may not reflect the market standard, leading to a conflict between looks, flavor and ultimate quality.

Judging perfection

“With harvest timing for apples, the big problems revolve around color and advanced maturity,” said Renae Moran, tree fruit specialist at the University of Maine Cooperative Extension. “Highly colored apples are worth more than poorly colored ones. In order to get a greater number of apples with good red color, growers need to sometimes delay harvest, which leads to advanced ripening and poor storability.”

Red color development in apples can happen at an earlier or later stage of maturity, depending on the temperature. Hotter weather will delay color development, causing the apples to be more mature at harvest.

“It’s always a compromise between storage life and quality,” she said.

While red apple color often determines when the crop will be picked, it’s the green background color of fruits that shows maturity. In the bulletin, “A Guide to Harvest and Storage of Tree Fruit in Maine,” Moran writes: “Ground color is a good indicator of fruit maturity for many varieties of apple, pear, peach, apricot and yellow plums. Areas of the skin with no red or purple coloration display the ground color or degree of green color. The skin contains chlorophyll which breaks down as fruit ripen so fruit display a change from green to yellow.”

Color development in other fruits corresponds to maturity. In plums and pears, “flesh firmness seems to be the best indicator, but color and flavor can be judged at the same time,” Moran said.

According to Desmond Layne, peach specialist at Clemson University, “Skin coloration has two different aspects,” background and blush, he writes in his “Everything About Peaches!” The change in background skin coloration “will typically change from green to yellow or orange as fruits ripen. Blush is the red coloration that occurs in response to sunlight.”

Harvest indicators

Like apples, peaches and plums destined for cold storage should be picked at a less than fully mature stage. Ripening has to do with ethylene levels. Picking prior to the rapid spike in ethylene production is key to storage quality. But some varieties of plums suppress ethylene, and if picked too soon, won’t ripen at all. Not only is it the type of fruit, but also the variety that often determines the best harvesting windows.

Measuring the amount of ethylene present to determine the right time to harvest a crop takes sophisticated instruments. Without going to this expense, growers can utilize a few other means of assessing crop ripeness.

For apple growers, Moran recommends the starch index to determine harvest timing. When utilizing the starch index – which is only for apples – an iodine solution is sprayed on a cross-section of the apple. The amount of stain that adheres indicates the levels of starch in the fruit. More stain means more starch. As apples ripen, the starch at the core begins to convert to sugars first, followed by starch in the cortex. Determining the optimal starch indexes for your market, and picking accordingly, can bring good results, with some precautions.

“In most years, the beginning of the rise in ethylene corresponds with a certain level of starch breakdown that is characteristic of each variety,” Moran said in the Extension bulletin. “Heavy crop load reduces the amount of starch in fruit and can lead to poor interpretation of starch index.”

Peach ripeness is indicated by a change in fruit size, which increases as the fruit’s cells expand in the two weeks prior to harvest. Crop load and irrigation, as well as variety, will affect the final peach size. This swelling, as the harvest window approaches, is a good indicator of maturity.

Firmness testing can be applied to apples, peaches, pears and plums. Instruments, such as a penetrometer, appropriately sized for the type of fruit, are used to apply pressure. The amount of pressure needed to puncture the fruit indicates firmness, which correspond to degree of maturity.

Firmness, starch levels and background color are often said to be the best indicators of maturity for most growers to use. There are, however, other methods that can offer insight into harvest timing. The days after full bloom, or DAFB, is a tool that predicts the harvest window and is enhanced as more years of data are available. Bloom is recorded by block and cultivar, and the timing of bloom, as well as the harvest, helps to establish the DAFB as a reference point for harvest timing.

The percentage of Brix, or sugar, is measured with a refractometer. As fruit ripens, the Brix levels increase. Weather pattern and growing conditions – temperature, amount of sunlight, moisture and crop load – will impact Brix readings, as will fruit position on trees.

“I consider Brix to be more indicative of fruit quality and flavor than fruit ripeness,” Moran said.

Taking measure

When testing fruit, take samples from healthy trees, and use fruits with no visible signs of damage. Select trees representative of the overall crop load. Fruit from the tree’s periphery is most mature and should be used for testing. Complete tests within a few hours of picking, as fruit temperature can affect results. The same principles hold true for testing tree fruits other than apples.

“Ten fruit will give a better maturity assessment than three fruit,” Moran said.

No matter what indices are being used, growers need to start monitoring and measuring well before the anticipated harvest window.

“To get better results, growers should start a few weeks in advance of the anticipated harvest to get a sense of starting values and rate of change in the starch index, fruit firmness and color development,” Moran advised. “They should also use taste tests and their own experience, since there can be considerable variability between different orchards. Commonly used maturity tests are not precise, so a grower’s own experience may be the best test.”

Knowing the degree of ripeness when picking the fruits of your orchard is part science, part art and a good dose of experience.