A few days ago I was doing my shopping at my local Sainsbury's when something very strange happened - please bear with me, I know it's not the most promising opening but I'll try to keep the clichéd middle class ranting to a minimum.

I'd got to the tills, and like the good little environmentalist that I am dug out the re-usable plastic bags that I had on me. It was at this point that the cashier noticed a miniscule tear in the side of one of the bags and, before I could work out what was going on, proceeded to rip a giant hole in the bag with the well-intentioned words: "you can't be having that, it might split".

Spotting the look of something pretty akin to horror on my face, she kindly offered to let me have another reusable bag free of charge - an act of generosity that while welcome only served to highlight the fact that a perfectly usable, if rather battered bag had just been destroyed.

I was too polite to point out that I wouldn't need a new bag, free or otherwise, if she hadn't just ripped my previous bag to shreds, nor that the only reason I had brought said bag with me was to avoid having to use another one.

Like many supermarkets, Sainsbury's has invested heavily in initiatives designed to cut customers' bag use, such as the provision of reusable bags and offers of reward card points for people who don't use disposable bags. I don't doubt the company could trot out countless statistics to prove this approach is working and judging by the number of people you see clutching the supermarket's orange reusable bags they are probably right.

But unless the people at the coal face who are actually tasked with executing these initiatives are fully aware that the whole point is to reduce the company's environmental impact and not just dish out a different type of bag their success will always be limited.

You can see this disconnect between HQ's green master plan and the shopworkers who have to make it a reality in Sainsbury's recent pledge to remove disposable bags from its till points.

In theory, this is a sophisticated policy designed to offer shoppers a gentle nudge in the right direction by forcing them to ask the cashier for any free plastic bags they want to use. The result being that customers have to think about how many bags they use every time they do the weekly shop.

In practice, at my local Sainsbury's the policy means that many of the people working on the tills simply put out a pile of plastic bags at the end of the conveyor belt ready for the next shopper, leaving the incongruous site of a stack of lurid orange bags beneath a sign explaining why Sainsbury's has decided to remove plastic bags from this very spot.

Other retailers are equally guilty of failing to ensure their plastic bag policies are followed diligently.

M&S has done more than most to combat plastic bag use with the introduction of charges on plastic bags for food, but the undoubted success of the strategy has been at least partly undermined by the introduction of smaller "sandwich bags" that are still free.

Consequently, a typical sight at my local M&S is people taking two smaller bags instead of paying for one larger bag. I asked one of the staff if this was allowed the other day and was told: "I don't think so, but we haven't been told otherwise."

There is a wider lesson here for all businesses, not just the retail sector.

The simple fact is that green policies are only as effective as the people who carry them out and unless staff are trained, motivated, and ideally incentivised to make them a success then they will always falter.

Sainsbury's and M&S have both made great strides in their battles against the plastic bag, and I don't doubt efforts have been made to educate employees about the various strategies now in place. But it is clear from interactions with staff in their stores that a good number are not really clear on why the policies have been introduced or what they aim to achieve.

It's a horrible term, but unless you can get "buy-in" across the business green policies are always likely to be prove less effective in practice than they are in theory.

I don't doubt plenty more plastic bags will be ripped up before this message gets through, but there is a strong case for spending just as much time and money on ensuring staff understand green policies as it is on developing those policies in the first place.