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A dish best served not.
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Typical plants (including most weeds) have green leaves and
stems. That color of light is reflected, while others are absorbed.
Some of the absorbed colors contain energy that the plant is able
to tap, to power the chemical reactions that keep the plant alive.
Most other green things also
are green because they reflect that
color and absorb other colors. Well, suppose we spray-painted a
weed with green paint? The PAINT would absorb the colors that
the plant needs, in order to survive! So, after a week or two of
the plant being in the dark, despite broad daylight, it will die. It
may die even faster if the paint blocks its respiration pores.
Now all we need is a non-toxic paint. I have linked a suggestion.
Simply check the weather before applying so that it can dry
before getting rained on. This particular substance will gradually
wash away in the rain, but it takes rather longer for that to
happen after it dries.
Something non-toxic that should stick
If colored green, it won't be obvious that the weed is getting killed [Vernon, Jul 11 2015]
Has toxity info. [LimpNotes, Jul 11 2015]
Hard to believe but they made my green weeding robot!
[pashute, Jul 14 2015]
No really, this is too much!
[pashute, Jul 14 2015]
||The problem with this is that you would have to
cover the entire weed. Also, most weeds will
recover even if you cut off everything above the
ground, so it's unlikely that this would work.
||Most weeds can be destroyed if you cover the ground
with black polythene and leave it for several
_weeks_, but even then some will regrow from their
||[MaxwellBuchanan], I tend to expect that when the spray is
applied, the ground around the plant will be covered, too.
And as indicated, how long the cover lasts depends upon
||Looking at the linked pigment database I was intrigued by copper resinate. This could be dissolved in a organic solvent that would evaporate leaving a glaze on the plant that wouldn't wash off. Copper salts are kinda nasty health-wise though so probably not a good idea, but just a thought.
||This is a simple enough experiment to do. Let us
know how it goes.
||There is a germ of a sci fi concept here: to want to
kill something but to want the thing to look good,
robust and healthy. Not obviously dying as it dies.
A sci fi or a murder mystery.
||Won't every species eventually be tagged for death by a unique metabolic pathway from it's chromosomal variation?
||Either technological design or nature's relentless subtle chew.
||It's a bit of an assumption to say that green paint will absorb everything except for green light*. You may find that it absorbs some frequencies, but is transparent to others - which will pass straight through to provide life-giving energy to the leaves of the weed you've just spent ages painting.
[*] Quick thought-experiment: Is cheap green paint likely to be opaque in the IR, UV, or X-Ray spectra?
||Make a small robot with a camera and web access to plant
recognition websites. (We have at least two in Israel). Have a
whitelist and blacklist of plants, and if unidentified send to
the weeder company for decision. The robot runs on methane
from dry weeds composted in its stomach and solar panels.
||Don't forget to paint it green.