FREEDOM — Just a few years ago, if kale was seen anywhere, it was lurking on restaurant plates and salad bars as a garnish or hiding in the kitchens of hard-core health nuts.

What a difference the years have made. Kale’s curly green leaves have become ubiquitous in the United States, equally at home, farm stands, urban smoothie bars and in the pages of magazines that tout it as a superfood. That popularity has put a strain on the seed suppliers, which last year ran short, causing local companies and farmers to scramble. And the shortage may not be over, according to recent news reports and an official from Johnny’s Selected Seeds of Winslow.

“As far as our business goes, it’s frustrating,” Pete Zuck of the Maine seed supplier said this week. “There’s this huge demand for our product, and we can’t satisfy that demand.”

About 85 percent of the commercial kale seed sold in the United States comes from a Dutch supplier called Bejo, which produces the best hybrid varieties, Zuck said. According to a July report from ABC News Australia, that company has run out of its stock of kale seed in Australia.

Mark Overduin, managing director of Bejo Seeds USA, told NBC News last week that the American supply is “very, very tight.”

In addition to the sharp increase in popularity in recent years — Overduin estimated that the amount of kale grown in the United States has increased nearly 60 percent since 2007 — kale also has some complicating factors.

One is that it’s a biennial crop, meaning that a plant must be in the ground for two years before it produces seed, and Maine is too cold for kale to successfully overwinter, according to farmer Polly Shyka of Village Farm in Freedom. Additionally, Bejo sells very popular hybrid varieties that are proprietary to the company, and seed couldn’t be saved from those plants even in the right climate, she said.

According to Zuck, another compounding factor in the kale seed shortage is the popularity of baby greens and microgreens. Microgreens are less than 14-day-old seedlings that are used in salads, soups and more. Kale is a popular pick for microgreen mixes.

“With those, you’re burning through seeds really quickly,” Zuck said.

Johnny’s employs about 200 people in Albion, Winslow and Fairfield, and it sells to customers all over the world. The Maine company gets a lot of its kale seed from Bejo, and most of its farming customers put in their kale orders in the wintertime. There was a problem this year, Zuck said.

“We just started running out of kale. One variety after another,” he said.

For Maine’s mixed-vegetable farmers, the kale seed shortage meant more work.

Prentice Grassi of the Village Farm said he had to scramble this year to find enough seed to meet the farm’s needs.

“We got scratching around for kale seeds this spring,” he said. “We ended up securing enough, but it was work.”

On the 5-acre farm, where he and Shyka grow mixed vegetables, herbs, chickens and more, the rows of different types of kale are vibrantly colored and healthy-looking. Grassi said they require about 3,000 kale seeds a year.

“For us, kale is one of the bigger crops,” he said. “We grow so many things that we can live without it — but it’s definitely in the top 10.”

Because the seed supply was so hard to come by this spring, that when the farmers planted it, they were on “lockdown,” Grassi quipped.

“Every seed was valuable,” he said.

Efforts to reach an official at Fedco Seeds in Waterville on Friday and Monday were unsuccessful, but a glance through the online catalogue showed that all the company’s kale seed was listed as sold out.

Zuck said that seed suppliers such as Johnny’s hope that the shortage will be alleviated soon, but it is hard to know just when that will happen. Bejo and other producers have honored their pre-shortage seed prices, he said.

In the meantime, folks at Johnny’s and other seed companies hope that other cruciferous vegetables in the same brassica family as kale will catch on with growers and customers. Though kale is incredibly high in vitamins A, C and K — with one cup of the chopped leafy green providing 684 percent of vitamin K — other brassicas such as collard greens, brussels sprouts and broccoli are no slouches in the health department. Johnny’s also is selling a new hybrid called Kalettes, which grows like brussels sprouts but instead of little cabbages, it features miniature kale plants.

“Nothing better exemplifies the emerging trend of innovative culinary delights,” Zuck said, adding that the company’s sales team was desperate to deliver kale to its customers. “I think our customers understand what’s going on.”

Grassi said that the scramble for kale seeds served as a reminder that small farms such as his are tightly connected to big seed growers, such as Bejo.

“You realize you’re vulnerable,” he said.