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#1
Old 04-08-2005, 02:31 PM
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Why doesn't weed killer kill my lawn?

OK, maybe this is just a really silly question but as I was administering some judicious squirts of chemical death the other day I got to wondering, how come weed killer only kills specific plants and doesn't affect my regular lawn grass?

I assume that there's something about crabgrass and dandelions and whatnot that they process some chemicals differently than generic lawn grass, it just struck me a little funny...are the various weeds really that different?

Is this the floral equivalent of chocolate being poisonous to dogs but not people?
#2
Old 04-08-2005, 02:47 PM
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Quote:
Originally Posted by Valgard
I assume that there's something about crabgrass and dandelions and whatnot that they process some chemicals differently than generic lawn grass, it just struck me a little funny...are the various weeds really that different?
It depends on the weed killer you are using, but yes, they do process the chemicals differently. Grasses are usually monocots and most weeds are dicots. Among the differences between these two groups is their different vascular systems. For some reason dicots take up certain herbicides (like auxin analogues) more rapidly than monocots. So, the lawn grass doesn't get a big enough dose of the herbicide to have an effect.
#3
Old 04-08-2005, 02:56 PM
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Pretty much. 2,4-D is a growth accelerant for broadleaf plants such as dandelions and plantain (not the bananas) and causes the to attempt to grow faster than their roots can sustain them. I don't recall the specific function of 2,4,5T, but it is specific to clovers and oxalis and similar runner-type plants.

Crabgrass is harder because it is in the grass family. The older methods were to put down a pre-emergent killer that knocked off the seeds (crabrass is an annual, and so needs to reseed itself each year while bluegrass, fescue, and longer term rye grasses found in lawns are perennial and do not need any seeds that actually fell before you mowed them off). One problem with the preemergent approach is that it must be applied before you have three consecutive days of 80 F weather, after which the seeds have already germinated. In the really old days, existing crabgrass was spot-killed with arsenic, then the resulting brown spot was reseeded with "good" grass. I have seen products to control existing crabgrass in recent years, but I'm out of the business, now, and I do not know their specific properties.

(Actually, since I got out of the business, 2,4-D and 2,4,5T have probably been replaced, but I'm sure the same principle applies--specific toxins for specific families of plants.)
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Old 04-08-2005, 04:03 PM
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And, if you're so inclined, you can find herbicides that will kill grass and weeds (and everything else) all in one go.
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#5
Old 04-08-2005, 04:37 PM
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Monocots and dicots refer to cotyledons, which are basically the first leaves a plant develops. Frasses, being monocots, send up a single leaf, while dandelions send up a pair (the true leaf stage comes later).

We can get into a nice discussion about root structure and uptake receptors and all that, but let's boil it down to the basic elements.

Grasses and weeds are two different families of plants. Because of that they are susceptible to different things.

You can also get herbicides that, for example, interfere with a plant's ability to use its cholophyll. That would kill everything green. You can also get herbicides that are more active on some species of the zea (grass) genus than others. They would, for example, kill sorghum while leaving its close relative corn basically unharmed.
#6
Old 04-09-2005, 02:09 AM
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You can also get herbicides that are more active on some species of the zea (grass) genus than others. They would, for example, kill sorghum while leaving its close relative corn basically unharmed.
Sorghum isn't a member the Zea genus. It's a member of the Sorghum genus and not at all closely related to Zea.
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