I use a surfactant even when rain isn't expected simply for the fact that it does provide better results, especially on weeds with a waxy coating. I'll spray weeds with rain coming within an hour with a surfactant and still have great results. It's a relatively cheap product that does work.
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I use an adjuvant only when the label calls for one. Adjuvants don't change speed of uptake -- they just change the physical properties of the water droplet or act as a solvent.

Research has shown us that phenoxy herbicides (2,4-D, MCPP, MCPA, dicamba) are usually taken up within 10 minutes after application and that addition of a surfactant doesn't affect the amount taken up or the time it's done in. The ALS-inhibitor chemistries often benefit from a surfactant, but the pnenoxies don't.

I'm really not sure what rain would have to do with it, especially at the 4-hr mark.

 

Adjuvants don't change speed of uptake -- they just change the physical properties of the water droplet or act as a solvent.

Droplets formed on the surface of a waxy leaf have less surface area in contact with the leaf than a fully wetted surface. Surface are ABSOLUTELY affects the speed of uptake.

BTW, chemicals absolutely can also affect the rate of transfer through a membrane. Higher concentrations lead to higher osmotic pressure. How an adjuvant enters into this equation can be complicated though.

 

I use a surfactant even when rain isn't expected simply for the fact that it does provide better results.
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This.

 

True story. I moved last fall to a new house with a 4 acre yard. Yeah...I'm not treating that! After my wife complained about the massive clover and dandelions I said I'd run over it with some 3way real quick. 1oz/K with a splash of chemstick, 45psi on the Z spray, and as fast as I could go. Dandelions curled over in a day and the clover is on its way out 5 days later. Moral of the story? I've spent endless hours on here reading and discussing precise ratios of concocted herbicide creations and every time I need to wipe out a neglected yard it's the ole 1oz of 3way and call me in the morning.
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Yeah but your clover and violets will be back with cousins next month

 

We use surfactant every day along with 4 - way weed control. If we see rain an hour away on the radar, we stop spraying. A surprise rain cloud hit us one day on a big property. We finished it up in a soaking rain and the weed control worked great.

 

We use surfactant every day along with 4 - way weed control. If we see rain an hour away on the radar, we stop spraying. A surprise rain cloud hit us one day on a big property. We finished it up in a soaking rain and the weed control worked great.

That's because as dew points and humidity rise with the upcoming rain event, plant pores open to accept moisture.

Years ago when I worked for Chemlawn, our branch agronomist told us the absolute best time to spray weed controls as far as plant uptake was in a light drizzle....however, we would never be able to convince the customer of that.

 

The following quote is from May 15, 2015 issue of Maryland Landscape & Nursery IPM Report.

Cassandra Swett and Kari Peter, Extension Fruit Pathologists, wrote these comments:
There have been several small rain events in the evenings with threats of larger rainstorms that have, up to this point, failed to manifest. With rain events comes the question: is it better to spray before or after it has rained? The conundrum: if you spray before, the plants are better protected but the product may wash off. If you spray after, the plants may get in infected before you can get in to spray, but you know that the product is there. The take home message from this article: it’s harder to wash off chemicals that you might think. Although a lot gets washed off, what sticks around really sticks and still works. It takes 1-2 inches to wash off enough fungicide to see a reduction in disease control, according to Dr. Annemiek Shilder’s studies at Michigan State.

 

Finished up a 35 acre job last week with storm clouds on the way. Last acre I sprayed in rain then it downpoured for five minutes.

Used three way with spreader sticker and got excellent kill on ancient dandies, dock, and plantain..

Also.....don't sell systemic control short.


I simply do not care what the clients think.....I'm too old for that, but the only reason I don't spray in the rain is because I get wet!!!


Weed control works in the rain people. Get over it and quit trying to come up with more excuses to get out of working.

Too windy too rainy too cold too wet too big a pussy.

Spray it.

 

Yeah but your clover and violets will be back with cousins next month

Doubtful. We'll see though
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True story. I moved last fall to a new house with a 4 acre yard. Yeah...I'm not treating that! After my wife complained about the massive clover and dandelions I said I'd run over it with some 3way real quick. 1oz/K with a splash of chemstick, 45psi on the Z spray, and as fast as I could go. Dandelions curled over in a day and the clover is on its way out 5 days later. Moral of the story? I've spent endless hours on here reading and discussing precise ratios of concocted herbicide creations and every time I need to wipe out a neglected yard it's the ole 1oz of 3way and call me in the morning.
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Back when 3 Way was labelled for St Augustine the Rate was 3/4 oz per thousand. I got a slower but good kill with only a 1/2 oz. 3 Way is no longer labelled for St Augustine.

.

 

Back when 3 Way was labelled for St Augustine the Rate was 3/4 oz per thousand. I got a slower but good kill with only a 1/2 oz. 3 Way is no longer labelled for St Augustine.

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I still occasionally use three way in the winter on St Auggie on a new start that's over run with weeds. It's still labeled for St Aug but only if it's dormant. I imagine down your way it doesn't really go dormant. We' usually have a month or two of dormancy but last winter we were pretty mild and didn't get it.
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I divided a small batch of old style T-Zone. One 16 ounce spray bottle had 5 ml Turbo (80/20) spreader-sticker(surfactant) added, the second had no additives.
I sprayed several pairs of weed species for comparison.

There was an obvious difference in the greater wetting effect of the surfactant-included solution. I am not sure if this will translate into greater herbicidal effectiveness. Results in a few days.
Photo shows some elm tree seedlings. Upper photo plain T-Zone. Lower photo T-Zone with surfactant (wet).