Don Boring owns a grocery store, a liquor store and now, a medical marijuana dispensary. The main difference among them is that he has to produce his own pot inventory.

Colorado set a Sept. 1 deadline for dispensaries to show they grow at least 70 percent of the pot they sell — the first requirement of its kind in the 14 states, along with Washingon, D.C., that have medical marijuana laws. Lawmakers added the requirement to Colorado's new law in hopes of keeping small-time caregivers from growing pot in their basements to sell to dispensaries.

Boring knows the types well. When he opened Annie's Dispensary last spring, he got so many visits from caregivers trying to sell him pot that he came up with a name for them — "guys on bicycles with backpacks."

Boring doesn't see those guys anymore.

"The days of the guys on the bicycles with a backpack selling marijuana are over, and I think that's a good thing," Boring said.

Lawmakers who supported Colorado's new pot law hoped the grow-your-own requirement would force shadowy corner pot shops to close and alleviate fears that the marijuana fueling Colorado's pot industry is coming from illegal sources. Lawmakers wanted to keep better track of how medical marijuana is produced.

But dispensary owners complain the growing requirement is confusing and impossible to enforce.

For one, the law isn't clear on how pot shops arrive at the 70 percent figure. Is it 70 percent by weight? Is it determined by the month? By the day? And what about pot shops barred by zoning from adding a growing operation? The law isn't clear, and pot shop owners have a lot of questions about how the growing requirement will be enforced.

"It's the equivalent of requiring a grocery store to produce 70 percent of its own corn. You're asking a retailer to also become a producer," said Brian Vicente, head of Sensible Colorado, a marijuana-legalization advocacy group. Vicente serves on a state advisory panel trying to clarify how the pot law's requirements will work. "Nothing about this requirement is clear," he said.

State authorities say it could be a year before pot shops know how the growing requirement will be measured. A month ago, Colorado received 809 applications for marijuana center licenses, though state officials say it will be next July before it will start awarding the licenses.

Matt Cook, the senior director for medical marijuana enforcement for the Colorado Department of Revenue, said the state received an additional 309 applications for "infused product" manufacturing — think pot brownies and such. The state also received 1,219 permit applications for "premises cultivation," or growing pot.

All combined, the state has collected $8.2 million in application fees for the three types of licenses, Cook said, with the money to be used to set up a new state office to regulate the nascent pot business.

Dispensary owners say the state must be joking if it thinks it can keep track of all medical marijuana.

Take a single marijuana plant, Boring explained. Some plants produce one ounce of smokable marijuana, while others are capable of producing 10 ounces or more, depending on how well they're cared for. So what keeps an unscrupulous marijuana center owner from claiming to be raising more pot than he is?

"How are they going to know? Are they going to come check on my plants every day? It doesn't make sense at all," Boring said. "It's just going to be impossible to enforce."

Veronica Caprio, owner of 420 Highways in Idaho Springs, complained that the vagueness of the state's growing requirement forces many dispensary owners to lie. She says many are still mapping plans to grow enough pot to supply their patients.

"It's too broad for me to honestly answer," she said of the growing requirement. "Do I think I can get to 70 percent? Absolutely. Can I produce it right now? Hell, no."

Danyel Joffe, a Denver lawyer who specializes in advising medial marijuana business owners, says other states are watching to see how Colorado attempts to enforce its growing requirement.

"No one has tried anything like this before on this scale," Joffe said. "We're going through the birth pains of a whole new industry. I'm telling my clients to grit their teeth and stick through it."