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An important practice in toxin-free (and other) turf care is to only mow with sharp blades. So how often do I have to sharpen the blades?
I'm sure that it will depend on conditions, but is there some general guideline? Is it based on area mowed? Hours mowed? After so many months?
And are there industry standards that professionals follow?
Depends on the mower you're using - if its just an ordinary electric hover mower with a rotary blade, or a non hover with a rotor blade, you just buy a new blade when its blunt or damaged, unless you want to put it in for a service every year, when they might do it if it needs it. If you can find someone to sharpen it, or you can do it yourself, fine, but in the UK, unless you sharpen it yourself, you won't find anyone to do it - replacement rotary blades for a flymo, for instance, are around a tenner, or £10, so not expensive, and I've only had to buy 2 blades in 30 years, mostly because they were either bent or badly chipped.
If you've got a cylinder mower, specially if its got an engine rather than being electric, its usual to have those serviced yearly by a lawn mower servicing company. The service may, or may not, include sharpening blades, that's something you'd have to ask them about. Most professional gardeners with large petrol mowers are quite capable of keeping their mowers functioning well, but not always including sharpening - if the mower is heavily used, it will be put it in for a full service yearly as a minimum. Groundkeepers are probably even more rigorous and are often capable of stripping down and replacing parts on their mowers, as well as sharpening.
As for how you know when the blades need sharpening, its impossible to predict - there's a big difference between a mower being used every day for 4 or 7 hours, and one that's only used once or twice a week. Damage may occur to the blades from stones or other objects hidden within the grass, and may cause the blades to be warped, chipped or simply blunt in parts.
If you're cutting grass all day long with a petrol cylinder mower, it'll likely need sharpening in spring and probably mid season. Otherwise, the easiest way to tell is by looking at the grass after you've finished - if the cut is uneven, then the blades need attention. If you're using a cylinder mower of any sort, inspect the top part of the cut grass - if it looks cleanly cut, then the blades are sharp, but if the ends look a bit chewed or tatty, then the blades need attention. This doesn't apply with rotary blades - they don't make a clean cut in the same way.
It depends on a lot of factors but for rotary mowers, every 20 to 25 hours is a good interval to sharpen your blades. For some people that means once a year, for others twice a year. For commercial landscapers it's once or more a week.
You can also just look at the blades of grass the next day or so after mowing. If the tips have a large (1/4" or so) ragged and brown looking area instead of just a small line of brown then it's past the time to sharpen your blades. Dull blades leave cuts that don't heal as quickly, are easier to be affected by disease and require more water than lawns mowed with a sharp blade.
I keep 2 blades for my mower so I can swap them out and sharpen at my convenience. When I started sharpening my mower blades twice a year it seemed to make a big difference. The sharpening before summer seems like a good idea. Learning how to sharpen your mower blade isn't hard nor is it time consuming. There are tools available that can make it easier and faster too. Though mower blades aren't that expensive I find it more convenient to sharpen than to purchase a new one.
I have some more info on my site in the articles Time to Change Your Mower Blade and Mowing Tips for a Healthy Lawn if you're interested.
Great answer, Bamboo. The old but best-to-cut-grass-with reel mower is easy to keep the blades sharp with a file or stone. Rotary blades need to be removed to sharpen correctly. At the very least make SURE THE SPARK PLUG IS DISENGAGED before messing with that blade!!! Rotary blades DO need to be sharpened regularly and I've gone and purchased extra blades for all rotary mowers. I take 2 or 3 in to the lawn equipment service store to have them professionally sharpened so all I have to do is change out the blade once per month or as soon as you see the grass tips getting ratty. This rattiness causes a 'dusty' look to ones lawn and could cause vulnerability to disease. If you know how to sharpen knives, pruners, hedge shears then great. DIY. Otherwise, they are not that expensive to keep extras professionally sharpened so change your blades out as often as you want! Don't forget to wipe with alcohol especially if you are doing maintenance on more than just your lawn! KEEP YOUR GRASS NO SHORTER THAN 3"!!! (if you have blue grass/fescue type lawn) Mow every day if you want but no less than once per week!
