On a weekend in early June, hundreds of families make the trek down a winding gravel road through the pine-covered hills near Black Forest to visit High Altitude Rhubarb, a small farm that grows 20 different varieties of the tangy, long-stemmed plants.

"People come with their children, and the children seem to enjoy the adventure of picking the rhubarb and being outside right where it grows," said Donna Duncan, who owns the business with her husband Dennis. "I wish more people would do pick-your-own farms. We get lots of questions about other farms they can visit."

Now retired from other jobs — Dennis, after working in computers and teaching college, and Donna, as a teacher and biologist — they've watched the rhubarb venture they started 12 years ago earn such a large following that most of their crop now sells out in a single day.

Ironically, rhubarb was something totally new to Dennis when they first moved to the six-acre property.

"A friend gave us rhubarb root," he recalled. "I'd never had rhubarb, but I researched and learned a little bit about it. When I planted it, it just took off. Friends told me the growth was exceptional, so I thought I would see what I could do with that. We were at a point where we could use some additional cash flow."

Rhubarb is something of a novelty. When the couple started out, Donna recalled taking fresh stalks to the farmers market, even bundling them up as pie packs, but feeling like what she had to offer was overshadowed by more common fruits and vegetables.

They had more luck shipping fresh rhubarb to enthusiasts living in warm climates, where the plant is difficult to grow.

But when they started doing the pick-your-own format, they found their niche.

"Our first customers would come up with 15 pounds of rhubarb they wanted to buy, and I would apologetically tell them the price, and they would say, is that all? And they would go out and get more," Donna said.

"Here at the farm there's this feeling that you buy it for the freezer, you buy it for your neighbors," she said during this year's open house.

"I just ran into a woman that is chopping it up and putting it in vanilla vodka to flavor it to give to friends. So you get to hear all the lovely things people do with it," she added. "That's the fun of having a u-pick operation."

One of this year's shoppers was Joanne Littau, of Denver, who runs a home-based business, The Jelly Jar LLC, which falls under Colorado's Cottage Food Act. In addition to using the rhubarb for jams and jellies, she planned to add it to a chilled strawberry soup swirled with fresh cream.

Interacting directly with customers is something the Duncans both enjoy.

Dennis recalled that early on Whole Foods Market had approached him and offered to buy their entire crop for the same price they were charging retail customers: $3.50 a pound. (They've since raised their price to $3.75 per pound.)

Dennis figured he would net $3 a pound on the sale, but the extra work involved in picking, boxing and delivery, not to mention hiring and maintaining a labor crew, was not enticing.

Selling the seasonal plants right off the farm has turned out to be a more satisfying approach.

"The harvest thing has turned into a big party," he said. "The customers are the ones who call it a festival, I think just because of the positive, happy atmosphere."

Fun aside, Dennis is serious about making genetic improvements to the crop.

He's responsible for a popular variety named Colorado Red, which he bred from plants he discovered growing in the wild.

"It turned out to be very unique," he said. "I decided not to patent it, but we have a contract with Gurney to sell it, and we do get a modest royalty from that."

He is now in the second generation of refining the seed.

"It will be a 15- to 20-year process to do this, so I won't be around when we get there, but I thought it was a project worth starting," he said.

Colorado Red is fairly small, producing around ten pounds of stalks per plant, compared to the more common Victoria variety, which can produce up to 90 pounds.

What makes it special is the intense red coloration that penetrates the flesh of the stalk.

"That has great appeal to jam makers and other people for whom color is important," Dennis said.

"Nutritionally they are the same (whether green or red) and there's nominal difference in flavor, but for some reason the market wants red," he explained. "That's a fairly recent development. Prior to World War II, 90-plus percent of rhubarb in this country was Victoria, which is a green variety."

Dennis currently trades seed with a leading rhubarb breeder in Australia, as the two each work to improve their own genetic lines.

"If we could get the productivity and health of the Victoria in a red plant, we'd really have something," Dennis said.

This year they sold out of rhubarb plants even before open picking weekend was held. And overall production was only half of normal, forcing them to limit picking to a single day. (They do accept visitors by special appointment.)

"The bomb cyclone in March buried our plants under two or three feet of snow, so they lost two weeks of growth there," Dennis explained. "Then they had another foot and a half dropped on them ten days before harvest, so that cost them another week. But with the hail risk, we didn't feel like we could postpone harvest."

Six years ago, they encountered a very different problem. The exceptionally warm, dry spring in 2013 triggered the massive Black Forest Fire, which took out an old horse barn on their property and was licking at the deck of their house before the wind shifted and firefighters were able to put it out.

Dennis describes the area's unique microclimate as "hostile to agriculture," due to hail, high wind, drought and a short growing season. Even so, the rhubarb thrives.

"It grows well here, and it doesn't take much water," Donna elaborated. "Rhubarb loves cold. In fact, it really needs the ground to be frozen for three months solid. And so far global warming has not messed up its down-time."

The Duncans, who are both in their 70s, are gratified to have created a distinctive and viable business worthy of passing on to someone else someday.

"We have everything you would need to expand the farm," Dennis said, noting that three acres remain undeveloped, and only about a tenth of the allotted water is being used.

"As far as this farm goes, I see only upside to it," he said. "Every year I have to turn down business."