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Welcome to the AOS Beginner's Newsletter. We will be sending you monthly tips on how to grow orchids and help you get them to bloom again. In addition to the information presented here, we invite you to visit the AOS website at www.aos.org and check out the information found under ORCHID INFORMATION > ORCHID BASICS.
Weeds
Right:
While ferns are lovely houseplants in their own right, they can easily spread throughout an orchid collection if not kept in check.
A weed is just a plant growing where it doesn't belong, although most weeds are also very prolific and multiply rapidly. They are also adaptable which allows them to grow under a broad range of conditions. In many instances weeds end up being labeled as "invasive species". Believe it or not, some orchids are considered to be weeds and in Florida and other semi-tropical areas of the US, some are on states' invasive species lists; Oecelclades maculata for instance. If you came to orchids from in-the-dirt gardening in USDA Hardiness Zones 3-8 you are pretty familiar with weeds. While dandelions, plantain and curly dock are unlikely to invade the orchids in your home or greenhouse (although they can be problematic if you summer plants outdoors), some northern weeds are the scourge of greenhouse orchid growers.
"So what's the big deal about a few weeds?"
Left:
Oxalis is not only a difficult pest to get rid of, it spreads rapidly and can quickly fill an orchid's pot with its own roots.
If a few weeds would remain a few weeds they would not be a problem. The fecundity of most plants regarded as weeds will quickly turn "a few" into "a lot". If weeds are allowed to become established in an orchid's pot and left to grow,
their roots will eventually take over the pot. While weeds will grow quickly on their own, they can be even more vigorous if you fertilize your orchids regularly as you ought to. Eventually the weed roots can completely fill a pot and use up all of the water and nutrients intended for your orchid. Additionally, the media will break down more rapidly. The weed will succeed while your orchid languishes.
While weeds taking over a single orchid pot are certainly not a critical matter,
many weed plants have the uncanny ability to disperse seeds over considerable distances. Some Oxalis species for instance, can shoot seeds for several feet when seed capsules ripen. Left untended, weeds in one orchid pot will become weeds in a whole orchid collection. When that happens, they can be extremely difficult to control. Seeds get down into the media, so just when you think you have a generation exterminated, new seedlings will be sprouting. The only way to eliminate a severe weed infestation in an orchid collection is to repot the whole collection...at the same time. Roots must be trimmed and rinsed under running water to eliminate persistently "sticky" seeds. Unless you have only a few orchids, repotting a whole collection in one fell swoop is not the way you want to spend a weekend. Like the other pests we have covered, and weeds are a pest, prevention and early control are the best solution to preventing a weed infestation.
Left:
A weed's roots can fill up and orchid pot and use all of the nutrients intended for your orchid. These roots also cause media to break down more rapidly.
Stopping weeds at the front door is the best way to prevent them from getting into your orchid collection. But it is not always easy to know whether a newly purchased orchid, or gift division from a friend
is weed-free. While weeds may not be visible above the media, seeds below may be ready and waiting to germinate. We like to put new orchids in a very conspicuous place for at least the first month. Seeing an orchid every day or so will alert you to latent weed roots or seeds because you will see them when they begin growth. Monitoring newly acquired orchids is good practice anyway to keep an eye on how they adjust to their new home. A large pair of tweezers can easily be poked into an orchid's media to grab a weed seedling with its roots intact. The best and easiest time to eliminate weeds is right after they sprout. The more developed their root system, the harder they are to pull out in one piece. Roots left behind will begin growth within a few days. If you do happen to leave roots behind when trying to pull a weed out of a pot, make a note to try again in a week when the weed has
developed enough to remove.
The best time to eliminate a weed is just after the seed germinates.
Keep a pair of tweezers in your growing area just for this purpose.
There have been some reports of people applying weed killers to weeds in orchid pots using cotton swabs or a paintbrush. This seems like unnecessarily risky business to us and we do not recommend the practice. No matter how careful you are, splashed poison can damage your orchids. Damage to orchids from weed killers can be insidious and you may not even see symptoms until long after you have forgotten using weed killer.
Should proactive weed control get by you, the only way to effectively eliminate weeds from one or a hundred orchid pots is to repot the infested orchid in fresh media paying particular attention to trimming dead roots and rinsing the old media off remaining roots under running water.
If you have been reading these newsletters for the past six months there should be one message that has been evident: treat little problems before they become big problems. Proactive pest control is in the end, far easier than eliminating an out-of-control infestation of insects or weeds.
Greg Allikas
March 2011
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