Irrigation methods control growth in high-density pears

Water management is used as a key tool in controlling vegetative growth at a high-density Mendocino County pear orchard.

Pears tend to be vigorous—frequently too vigorous—and there are no size-controlling rootstocks to help maintain the optimal balance between fruit and foliage. Too much water can cause vegetative growth that shades out the fruit, but too little water makes it difficult to achieve the size of pears desired by canneries.

The problem can be even more challenging in a high-density orchard, with trees planted just four-and-a-half feet apart and kept short enough to be worked from the ground. Water is one of the few tools that can be used to limit vegetation in these tightly spaced trees.

"We start the trees with just a single gallon-an-hour emitter to get them going. Then we put in a second gallon-an-hour emitter later. We've found that just one irrigation a week is enough," Chris Ruddick said as he pointed to a plot of high-density pears in his orchard outside Ukiah. He spoke during an irrigation field day sponsored by the local resource conservation district.

Just a single irrigation of 15 to 16 hours a week through the two, one-gallon drip emitters is generally enough for these high-density trees, he said.

"We use less water. We're focusing on just the base of the tree rather than the entire area. We still have sprinklers, but they are just for frost protection," Ruddick said.

This miserly irrigation through drip emitters does save water, but it also plays a vital role in keeping the trees open enough to produce high-quality fruit.

"Everybody is probably wondering what we've got here," Ruddick said, as he motioned to his tightly spaced pear trees. "The underlying thought is to get guys off ladders. We are trying to grow something short here and we wanted to stay away from metal trellises and keep it simple."

The trees are pruned and harvested from the ground, with just one worker following up with a short ladder.

The experiment is working, Ruddick said. The closely spaced trees are going into their eighth year, and since they became established the yields and bottom line have been excellent.

"Last year's production was 22 tons an acre. At first, the cost was the same as the other areas of the orchard because of the cost of training the trees. The last couple of years, the cost has been 10 percent less with the high-density section," Ruddick said.

His rootstock of choice for the high-density plot is OHxF 87, the smaller of the two common commercial pear rootstocks.

"The challenge for anybody doing high-density pears without a limiting rootstock is to maintain the sunlight going down into the trees, rather than just to the top," said Rachel Elkins, University of California Cooperative Extension farm advisor in Lake and Mendocino counties.

A few rows down from the high-density pears, Elkins planted a rootstock trial in 2005 in hope of finding a size-controlling rootstock suitable for commercial pear production. The trial is part of a national NC-140 rootstock study that began a quarter century ago.

"We don't have a size-controlling rootstock. The only thing we have is quince, which is not compatible with Bartlett, and the nurseries don't want to do an interstem," Elkins said.

No rootstock in the trial has shown yet that it can produce good yields of large pears, while also limiting vegetative growth.

Elkins and UC Cooperative Extension specialist Karen Klonsky showed in a 2006 study that high-density pears planted on precocious rootstocks can pay off.

In that study, the high-density plots had 1,244 trees per acre, compared to the standard 242 trees per acre. The cost for establishing the high-density plot was around $3,500 more per acre. But the higher-density planting more than made up for the upfront cost by producing better yields of marketable fruit.

Those results have been confirmed by the high-density planting in Ruddick's orchard.

"Chris has done at least 20 tons an acre since the fourth leaf. It's gone as well as we could have hoped. The costs of setting this up have been paid. We don't have too many new pear plantings, and it was nice to see someone take the risk," Elkins said.

While the search for a rootstock suited to this system continues, irrigation remains the most economical tool that can be used to control the growth of the high-density trees.

"Pear trees tend to be quite vigorous, and it's always been interesting to me to balance the tree, to see if we could use water to control growth. We're trying to correlate water use of pears with other attributes like the balance between vegetation and fruit," Elkins said. "We're just looking at opportunities to get some growth control and still have the size fruit we want."

There is a delicate balance involved when using water to control vegetative growth in pears. On one hand, the canneries want large pears and it takes water to size the fruit. On the other hand, the application of enough water to size the fruit will eventually lead to too much vegetative growth.

"Our costs might be back up as high as with the big trees this year, because we'll have to prune to keep the sunlight in," Ruddick said.

(Bob Johnson is a reporter in Magalia. He may be contacted at bjohn11135@aol.com.)