Why no weeds?

Here's a backward question. At my apartment complex, there are long stretches of flower beds where the only thing growing is the usual generic evergreens. I have not seen a single plant growing there since the ground thawed in April. I know for a fact that nobody's doing any weeding, but I *do* know that last September, some ancient man was walking around with a tank & sprayer, hosing down all the bare spots. Even so, I'd expect SOMETHING to pop up almost a year later. He had no idea what he was applying - "They just told me to spray what was in the tank".
This is just curiosity on my part, but what could he have applied that would kill everything for such a long time? By the way, there's no weed barrier in these beds - it's about 25% covered with some faded wood chips, but that's all.
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Many of these "generic" evergreens shed needles (or scales) that are preemergence herbicides and prevent weeds from coming up from seed. One good examples is juniper Virginianis. That is why some forests have few weeds on the forest floor.
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wrote:

generic
Really? Even 5-6 feet away from the shrubs themselves?
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It depends on the spread of needles or scales. The chemicals are released from the needles or scales. If they get scattered around, then the toxic effect is scattered around.
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But additionally plants like junipers sometimes grow in hard compacted organically depleted & dry soils that little else can get a foothold in. If the whole area had its soil loosened & some peat worked into it, & a bit of water gotten to the improved soil, there are a whole host of plants that actually thrive in the environment created by junipers or walnuts & other trees & shrubs that exude growth-retardants -- such as virginia creeper, daffodils, grape hyacinths, daylilies, cranesbills, sweet woodruff, & a god's plenty of weeds.
-paghat the ratgirl
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wrote:

One
few
I agree with this. Once you kill off all the perennial weeds and let the soil surface crust over, weeds just don't grow. If you break-up the surface and mix in some organics to prevent it from crusting over, all kinds of weeds will grow.
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wrote:

generic
This is not exactly true - allelopathy is not exactly the same as a pre-emergent herbicide, although the effects can be somewhat similar. And the lack of weeds or significant undergrowth in heavily forested areas is most often due to competition of the larger root mass of the trees for nutrients and soil moisture combined with persistant shade from the forest canopy. In more open areas, weed and native growth is generally pretty prevalent.
The lack of weeds in this case is probably a combination of factors. There are some pretty heavy duty herbicides out there which are commonly used in the maintenance of commercial properties plus many wood-based mulches have allelopathic properties of their own, albeit small and not very longlasting. Also, many pre-emergent herbicides can have a rather longlasting effect - upto about 9 months. That combined with the fact that most common weeds seed in fall or spring (but not in summer) can account for the fact that you haven't seen any significant weed development yet.. If no further spraying is done this fall, I''ll bet you see some weeds start to develop with the fall weed crop and the autumn rains.
pam - gardengal
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wrote:

There
longlasting.
seed
Hmmm. I'm glad I didn't plant any herbs or vegetables there.
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I wrote:

Allelopathy is a variety of chemical processes that plants use to keep other plants from growing too close. These compounds include alkaloids, cyanohydrins, sulphides, flavaniods, terpenoids, steroids, phenolic acids, aliphatic acids, glycosides, lactones, tannins, organic acids, purines, nucleotides, cinnamic acid and sugars. In one kind, the plant that is protecting its space releases growth-compounds from its roots into the ground. New plants trying to grow near the allelopathic plant absorb those chemicals from the soil and are unable to live. A second type of allelopathy releases chemicals that slows or stops the process of photosynthesis . An allelopathic plant may also release chemicals that change the amount of chlorophyll a plant has in it. When a plant's chlorophyll levels are changed, it cannot make the food it needs, and the plant dies.
Some plants that use allelopathy are black walnut trees, sunflowers, wormwoods, sagebrushes, and trees of heaven.
There are several ways in which an allelopathic plant can release its protective chemicals:
Volatilization: Allelopathic trees release a chemical in the form of a gas through small openings in their leaves. Other plants absorb the toxic chemical and die.
Leaching: All plants lose leaves. Some plants store protective chemicals in the leaves they drop. When the leaves fall to the ground, they decompose. As this happens, the leaves give off chemicals that protect the plant.
Exudation: Some plants release defensive chemicals into the soil through their roots. Those chemicals are absorbed by the roots of other trees near the allelopathic one. As a result, the non-allelopathic tree is damaged.
Some pine trees are allelopathic. When their needles fall onto the ground, they begin to decompose . The soil absorbs acid from the decomposing needles. This acid in the soil keeps unwanted plants from growing near the pine tree. It has been theorized that phenolic acids present in pine leaves may play a vital role in the allelopathic inhibition of understory species. Hence, many pine forests have very little on the forest floor other than pine needles.
Many junipers produce a phenolic compound found to inhibit seed germination. Such a compound is called a preemergent herbicide.
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