In San Diego the program that gives you a discounted electric mower for your old gas guzzler is so popular that the county prohibits residents from camping out overnight to get a prime place in line. In exchange for your old gas push mower and $100, you can get a new electric Black+Decker model that did pretty well in Consumer Reports' latest mower tests.

That's a switch from years ago when mower exchanges were first set up to offer consumers an extra incentive to give up their gas mowers, which spew emissions, for cleaner electric models. As we confirmed in our tests, early electric mowers left a lot to be desired when it came to cutting, so consumers needed a good reason to make the trade.

Today, things are pretty different. Consumer Reports' top-rated electric mower, the Ego LM2101, $500, offers performance on par with the top gas push mower in our ratings, the Cub Cadet SC100, $250. And lawn mower exchanges are a little different, too. Fueled by demand for modern electric mowers, many exchanges, which previously ran all spring, run just one day per year, or in some cases, for a few hours, until all the allotted mowers are gone.

At the San Diego mower exchange—which is entering its 18th year—folks start lining up at 4:30 a.m. and the gates open at 8. People arrive with their old mowers and leave with a Black+Decker CM2040, which in our tests was good at mulching, bagging, and side discharging, and very easy to handle. The Black+Decker usually retails for about $350. This year you can also exchange your gas-powered string trimmer, chain saw, and hedge trimmer, and get electric models for $80 each.

The same Black+Decker mower will be offered this year by the Colorado Regional Air Quality Council in Denver, where you can preregister for the events, which are spread out over three different weekend days.

Similar events are being held across the country. Though there's no central place that lists them all, to find one near you start by contacting your local environmental or air quality management department (names vary by state).

In addition, some mower manufacturers, including Black+Decker, post these events on their own websites. But keep in mind that as tempting a bargain as a lawn mower exchange might be, the deal is only as good as the mower that’s being offered.

Once you’ve found a local event, use our lawn mower ratings to see how the models being offered stack up.