Weeds can be a problem for orchids. They can compete for water and light, and their roots can make the orchid's potting mix break down more quickly than normal. Though epiphytic orchids don't grow in dirt, limiting the number of competitors that can coexist with them, there are a number of plants that can pose problems. Terrestrial orchids are vulnerable to a much wider variety of competing plants.
Oxalis plants growing in a Bulbophyllum lobii and several other orchids.
Personally, I've had the most trouble with ferns. They can be aesthetic, but they pull large amounts of water away from an orchid's roots, and the osmunda (or rhizomes) they produce can significantly change the composition of the potting mix, so that it's no longer ideal for the plant. Pull them out of the pot as soon as you see them! (If you like the fern, feel free to pot it up separately. A weed is just a plant out of place. But if you have a fern growing near your orchids, be prepared for it to distribute spores, which may lead to more ferns appearing in your orchids.) If a fern has gotten out of control, it may be necessary to repot the orchid to eliminate it.
Oxalis is another notorious invader of orchid collections. These plants look like large shamrocks, and their roots will quickly fill the orchid's pot and outcompete it. Also, the seeds spread widely, so early control is essential. Pull them as soon as you see them.
Other orchid growers have trouble with various other types of plants invading their orchids; some of them, like ferns, are nice plants in the wrong place. Others are just evil. Which enemies you'll have to contend with depends on where you live, but in general, yank 'em right away before they go to seed! If they get out of control, repot all the affected plants, preferably at the same time.
It's also a good idea to have a quarantine area for new orchids. Grow plants in a separate area from your main collection for at least a month, so you can watch for competing plants, pests, and other diseases. Choose an area where you can take good care of new acquisitions, and which is also conspicuous enough that you'll notice anything amiss.
Do not use herbicides to get rid of your orchid's competitors; there is too much risk to the orchid.
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