Eliminating established weeds in pots
 

Hi Everyone:
I have a large greenhouse collection of Cattleyas, mainly in a bark mix that have established oxalis and another broadleaf nuisance that I need to eliminate. Any ideas on how to do this without disturbing the established orchid? As you know pulling without getting the root is futile as they come back. I was thinking of mixing up a solution of round-up (no lectures please) and lightly applying to a few leaves of the weeds with a small soft brush and see if that works. Any other ideas would be appreciated.

Should work. I would give it a try. I've done it in a garden pond to get rid of a reed, another time a cattail that took up residence. Just start with a couple and see how it goes.

Carefull when you water as the herbicide on the leaves might be dragged to the medium.

Good point. I would definitely make sure you're using a Roundup that becomes waterproof quickly (some do, some don't). I'm no expert by any means, but it is my understanding it must be applied foliar. Once it's on soil, or water, it looses systemic action. Bark.... ummmm, I don't know, but it would seem to be the same reaction.

For example, I carefully sprayed over an area in a pond where I wanted to kill an invasive plant. There were a ton of other plants in the pond, plus a bunch of pricey koi. Nothing happened to anything other than to the targeted invasive plant.

I'd also caution to not try it full strength. Dilute as recommended before applying.

If using a brush, make it a small foam brush. A bristled one can flick droplets onto your good plants as you use them.

The only time I tried that, I used a q-tip.

Chalk up another advantage to semi-hydroponic culture, or at least the use of leca. I find that weeds pull out whole extremely easily compared to organic media.

Surgically-applied Roundup can work well without hurting the orchid, especially if it is a big, well established Catt. I know a commercial grower of Cyms who got away with using it not-so-surgically. The slow growth of the orchids is what saves them - the herbicide goes after things that grow fast - like weeds. Certainly keep it off the orchid leaves as much as possible, but if a little escapes it will probably not do much harm to the orchid relative to what it does to the weed.

True. I get no issues with my orchids, as they grow in volcanic rock. Real easy to pull out if weeds begin to grow. But, most of the time, or nearly all of the time, I don't get weeds.

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Definitely. A q-tip or those sticks with the little cotton buds on the end. Making sure that the end of the q-tip is moist.

Then just brush onto the leaves of the oxalis etc.

In Austrailia we have what are termed as "Selective Lawn weeders". Basically these are herbicides which target "broad leaved plants" more than they do "narrow leaved plants". In other words they target dicotyledon more than they target monocotyledon. Not sure how they work but they do seem to work (I just used one). Orchids are monocotyledon and so should, theoretically, be less targetted by these agents than, say, Oxalis (which is a dicotylon). The agent I just used has 200g/L of Bromoxynil and 200g/L of MCPA. I heard of people who used something like this on an orchid collection (commercial) but have not done so myself. The Q-tip\cotton bud method is the one I used.

Thanks to all who replied to my question.
I realized that Round up works through the leaf not if the soil. I suppose I could mix up a weaker solution a apply to one less desirable plant and see how it goes. My concern not so much the contact with pseudobulbs as the roots that have a covering of velamen that would absorb and hold this product.
Ill keep you all posted as to how my "sacrificial" plant fared. If it works, I will share the concentration and the details.

Some articles mention things about roundup taking 140 days or so in soil to break down to half strength. That's what it said anyway. I don't know much about roundup. It does say that plants can still absorb roundup from the soil.