SNRUB, I too appreciate the added info. I never knew, but it makes sense as sometimes I mix and spray and nothing happens, I mix stronger and spray and nothing happens.
2 f/up questions.
1. I could use RO water but my little house tank would take a while to make several gallons. In my garage I have well water (hard and with iron) I also have softened water, I assume that softened water might be better to use to mix with RU, but it just has different 'ions' in it instead of calcium carbonate right? If soft water is better then I'll mix with that, or I guess if I plan ahead I can spend a few hours setting aside gallons of RO water
2. IS there a temp requirement for RU to work? I'd love to use it now but daytime temps are probably too low? Why is temp important?
Do some searches and all that information is available. I have read most of that over my farming career but I just don't trust my memory to quote it correctly. I know the ammonium sulfate that we used to use to condition the water (and now we use a much more user friendly though more expensive liquid conditioner) tied up certain ions so that the ions would not tie up the glyphosate. But I can't remember without looking it up so I can't say for sure on the softened water. The salt in the softened water might tie up the RU worse. Just can't remember. Glyphosate is a salt, by the way. Table salt will also kill plants if you put enough on. It just does not take much of glyphosate salt.
Here is the easiest solution if you don't want to be a chemist. If you spray a fair amount of RU around your place periodically, simply go to about any coop or farm supply where they sell chemicals to farmers and ask for some water conditioner for use with Roundup. I'll go over to the shed tonight and get you a brand name but there are numerous brand names that work. It will be in a 2.5 gallon jug and it will last you for years (and it is cheap). You will not need a restricted use pesticides applicators license because it is not a restricted use chemical. In a three gallon hand sprayer just put your water in, put three or four tablespoons of the conditioner in, slosh the water around to disperse, then put the glyphosate it. Problem solved. The conditioner will have the right stuff to tie up the ions that are responsible for tying up the RU.
There are populations of weeds that have become resistant to RU. They gain this naturally over time by natural selection. We have populations of waterhemp and marestail that glyphosate will hardly touch it. It is nothing about the chemical causing the plant to change, it is all about natures natural selection process. Any plant that produces gazillions of seeds has the propensity to develop resistance much quicker than plants with low numbers of seeds. It is all about probabilities.
In any plant population of most if not all species there will be the one plant in a thousand or million or billion that will show natural resistance to a herbicide. So lets say in this particular field there is one plant in a billion seeds that have germinated that has this natural resistance to RU (or any other chemical for that matter). You spray the field but this one lone plant survives and produces seed. You have started the resistant portion of the natural selection process. Now if you rotate to some other crop or use mechanical cultivation to destroy it or use some other chemical to get it then maybe that is the end of that resistant population (the importance of mixing up chemicals and modes of action of chemicals and other means of control such as mechanical). But lest say you use only RU in that field for 10 years in a row and it is the only herbicide used. That one resistant plant produces seed. If it is a coclebur plant maybe it only produces a thousand seeds. But if it is a small seeded waterhemp maybe it has 10,000 seeds (see why small seeded plants develop resistance worse?). The next year you spray RU instead on one plant surviving the chemical (remember the RU is still effectively killing the bazillion other plants in the field successfully) ten plants survive. Then the next year a hundred. The next year a thousand plants and so on and so on. All the while the RU is successfully killing all the other plants just like it always have. But you notice each year more and more plants are not being killed. It is not that the plants have modified their construction or genetic make up to resist the RU. It has just been a process of natural selection where you have been killing the plants susceptible to RU while allowing the plants naturally resistant to it to multiply. Then at some point the plants that are naturally resistant out number the plants that are not. And you have a weed resistant to RU. It is not that the weed has changed. It is a natural selection process of naturally occurring resistance. That is why best management practices are used to keep the process happening as slowly as possible. Roundup Ready crops that resist RU originally were selected in the same way, from populations of naturally resistant plants within a susceptible population. Now they do it a lot faster because they can identify the genes that control the resistance and insert them artificially, vastly speeding up the selection process compared to raising many generations of crops.
Long winded explanation to tell you if you use the same chemical over and over and over and over again, you are naturally selecting for the plants that naturally resist it. Its all natures way of adapting.
Roundup does not work as well in cold temperatures. That is likely because most plants do not translocate nutrients as well in cold weather. But very susceptible plants (johnsongrass for example) will still get a good kill in cold (fall) weather. Plants that are tougher to kill with RU (winter annuals like henbit for example) do not respond very well in early cold spring weather. That is why RU has problems with the winter annuals in the spring. A combination of the chemical doesn't work that great on them to begin with and the fact that winter annuals main growth period is when it is in the cold parts of the year RU just isn't that great of chemical on those. 2,4D on the other hand will whack them (but it is volatile so you have to be mindful of your neighbors flowers or garden).
I can't say why your RU doesn't always work. There could be various reasons from the plant not being very susceptible, to resistant populations, to dirty carrier water inactivating the chemical.
If you leave RU in your sprayer over night, spike it a little the next day. The water may have already tied up the glyphosate. Best to use it all within a few hours of mixing. Distilled water will eliminate that, conditioned water will help it.
