By Anne Fisher
August 16, 2012

FORTUNE — Dear Annie: I liked your piece about taking work-free vacations, but the issue I’m struggling with right now is, what can I do about people who seem to be on vacation without leaving? We are on “summer hours” — everyone can leave at noon on Fridays, and some people also take Mondays off — but I have two employees in particular who take even more unofficial time off than that. One is preoccupied with children at home, and the other is always coming in late and leaving early to train for an Iron Man competition.

These are talented people, super-conscientious throughout the rest of the year, and I really hesitate to be Mr. Killjoy and start cracking the whip, but we do have deadlines to meet and things that need to get done regardless of what else is going on. It’s a temporary problem (for one thing, school starts up again soon, luckily), so should I just wait it out, or what? I’d be interested to hear how other bosses deal with this. — Greetings from Ocean City

Dear G.O.C.: It probably doesn’t help that your office is located in a beach resort town, but being at work while hardly working, sometimes referred to by human resources folks as “presenteeism,” is endemic everywhere in the summertime. Consider: About 20% of white-collar workers say their productivity takes a dive in the warm months, and 19% say attendance drops off, according to a recent survey by digital media company Captivate Network. Longer project turnaround times were reported by 13% of those polled, and 45% said they are “more distracted.”

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The survey also found that “the addition of summer hours only exacerbates these problems. For example, 53% of employees who leave early on Fridays report a drop in their own productivity, and 23% of those who make up for fewer Friday hours by working longer from Monday to Thursday report that their stress levels increase.” Says Mike DiFranza, Captivate’s president, “On the face of it, summer hours probably seem like a terrific idea and are welcomed by all, but unfortunately, the impact is almost uniformly negative.”

Randy Harrington, CEO of consultants Extreme Arts & Sciences and co-author of a new book, Evolutionaries: Transformational Leadership, has worked with companies like Microsoft (MSFT), Adobe (ADBE), and Yahoo (YHOO) on boosting productivity. “If you look around your office and see that your employees have ‘gone fishing’ or ‘at the beach’ written on their foreheads, it’s time to overhaul your summertime workplace culture and find new ways to energize people,” he says. He offers these five suggestions:

1. Recognize greater outside demands. “People with young kids are drawn out of ‘work mode’ more easily when school is out, and it’s a lose-lose proposition to make them choose between their jobs and their children,” Harrington says. “Sometimes a few simple adjustments can help a lot.” One of his clients, a major retailer, moved people’s shifts so that employees with kids could work more evenings, when a spouse or other adult was on duty at home. “You could also try a technological solution, like setting people up with Skype, so they can see for themselves what the children are up to,” he says.

2. Set the tone from the top. “If your senior leaders are wandering in late off the golf course and leaving early, that sends a cultural message that is hard to overcome,” Harrington notes. “And believe me, people are watching.”

3. Identify special projects. “Instead of fighting it, why not use ‘slacker season’ as a time to tackle some things you need, or want, to do that aren’t part of the regular workload?” Harrington suggests. “You can create ways for people to have some fun while producing something of substance.”

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One of his clients, a Florida credit union, earmarked four weeks in the summer for a cross-departmental team to put together a “culture book” — a corporate yearbook, made famous by online retailer Zappos — designed to give new hires a candid view of what it’s like to work there. “Some companies do a new one every year,” Harrington says. (Zappos’ culture books are so popular that they’re available on Amazon (amzn).) “It’s a great bonding experience for employees, since everyone has a hand in it.”

4. Don’t bother with “fake fun.” Special events intended to beat the summertime blahs rarely achieve anything, Harrington says. “Employees most often see these forced parties as a waste of company resources,” he oberves. “I actually saw a memo at one company that said ‘Lunchtime Luau: Attendance Mandatory.’ Fun is key, but it has to be authentic. Find ways to make work more interesting, and ‘fun’ will take care of itself.”

5. Establish commitments for people to work on professional development. “You need to continually develop your talent anyway, and the summer slowdown is a great chance to get it done,” says Harrington. “This may mean setting expectations that people will take outside courses, pursue online training courses, participate in reading groups, or start doing formal mentoring. If you encourage people to do some big-picture thinking during the summer, they start looking forward to it in the spring.”

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And speaking of spring: As you’ve no doubt noticed, most of these tactics require some advance planning. “The time to start thinking about keeping employees engaged in the summertime is really in March or April,” Harrington points out. “That’s when you need to have conversations with your people about goals and strategies for the dog days.” Too late for this summer, of course, but it might be smart to make a note on your 2013 calendar.

Talkback: Does the pace of work slow down in the summer at your company? Is that necessarily a bad thing? Leave a comment below.

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