For decades, Idaho's sugarbeet growers have been rotating their crops with potatoes. Now, a University of Idaho weed scientist says they may be unintentionally growing as many as 211 sacks of potatoes while they're raising sugarbeets.

According to Don Morishita, so many small, leftover potatoes from the previous year's harvest can sprout among the current year's sugarbeets that sugarbeet root yields can be sliced by 25 to 61 percent. "It was really an eye-opener for me," Morishita says. "I think what really makes the potatoes so competitive is that they have a jump on the sugarbeets early in the season and it's just hard for the beets to catch up after that.

"The sugarbeet roots and the potato tubers are competing for underground space, and there's just a certain amount of space that's available for them to grow."

Morishita decided to measure the potential impacts of volunteer potatoes on sugarbeet crops back in 2005, after learning that Washington State University scientists had found an average 9,985 leftover potato tubers per acre in fields they had surveyed. At the University of Idaho's Kimberly Research and Extension Center in 2005 and 2006, he deliberately planted potatoes in seven different densities-between 2,728 and 16,336 plants per acre-mixed in with sugarbeets.

On average, in a 100-foot crop row, Morishita's research team found 12 potatoes tucked inbetween sugarbeets in the plots planted with the fewest potatoes and 69 potatoes in the plots planted with the most, with resulting yield losses of 25 to 61 percent. Plots seeded with a roughly average amount of unwelcome spuds-8,168 per acre-left 34 potatoes within sugarbeet rows at a yield cost of 42 percent.

Currently registered sugarbeet herbicides have little effect on volunteer potatoes, Morishita says. The intruder potatoes produced tubers as large as 6 ounces-2 ounces more that it takes for a spud to grade U.S. No. 1.

Morishita also found that the best time for hoeing out an average number of weedy potatoes was when tubers were just beginning to form underground-about a month after plant emergence. Hoe before then and you'll soon be hoeing again, he says: the stored energy in the tuber will send up a new potato plant that's still capable of nipping sugarbeet yields. Hoe later and the potatoes will already have begun to take an unacceptable toll on the beet crop.

There are more reasons than simply eliminating competition to remove volunteer potatoes from sugarbeet fields, Morishita notes. They can host potato diseases, threatening neighboring potato fields as well as future potato crops.

Morishita will repeat the timing-of-removal portion of his "interference" study this year.