Lasers!

We've been investigating what it takes to kill a plant with a laser. We aren't the first to consider this method of weed elimination. A well-regarded laboratory in Germany, LZH, began research on this topic last year.

http://www.lzh.de/en/publications/pressreleases/2010/unkraut

But LZH is not that far along and we need to know the feasibility much sooner.

First question: How much energy does it take to kill a weed? There is decent research on energy required to kill weeds based on propane torches. It takes 40kg of propane to clear 1 ha of early stage (0-2 true leaf) weeds. Assuming about .25 square cm for a small weed and some amazingly efficient heat transfer, this works out to about 5 Joules of energy to kill the weed.

How much energy will our laser require? Well, we estimate that the laser needs to kill the weed in about 10 milliseconds if our device is traveling at 1.5 mph. That means the laser needs to be pumping out 500watts of energy at the upper limit. In reality, lasers can do some fancy "slice n' dice" manuevers ... and therefore we can probably get away with a 100 watt device. We'll have to do some testing to confirm.

What wavelength to pursue? Again, testing is needed. Based on our discussions with the CEO of nLight (http://www.nlight.net/) we know that the most cost effective wavelength is 800-900nm with a cost of somewhere around $20/watt. But water (main ingredient in a weed) best absorbs light at higher frequencies - think microwaves. Is the improved absorbtion efficiency worth the extra cost? Testing is required.

In the end, we're looking at about $2,000 to $5,000 for the laser and another $10K or so for the scanning equipment. Question is, will it do the job? Probably, but only after a weed zapping test will we know for sure!