No, it is not true. If you water plants with water that is too hot you can damage the roots, but that is a function of heat, not whether or not the water has been specifically exposed to microwaves. Microwaves do not change the structure or composition of water; all they do is add energy, same as boiling on a stove. Assuming you let the water cool, it behaves like any water.

Two science fair experiments have been carried out to answer this question experimentally, one in 2005 using cat-grass, and one in 2006 using a pair of plants with established root systems whose stems and leaves had been cut back at the beginning of the experiment so as to encourage the growth of brand new shoots or leaves. The 2005 experiment was a group project; a full report can be found at <http://www.virtualsciencefiar.org/2005/brun5r0/> but I will summarize the experimental design and results obtained here. The cat-grass was grown from seed, and the experiment measured the length of the grass blade (height of plant). There were 4 categories of water used: water that had NOT been microwaved, water that had been microwaved for 2 minutes, water that had been microwaved for 5 minutes, and water that had been microwaved for 10 minutes. (The microwaved water was allowed to cool to room temperature before it was used to water the plants.) The same measured amount of water was used for all plants, but the plants were watered from below by pouring the water into a container, and then setting the pot of cat-grass into the container of water. Each pot was consistently watered with the same type of water for a period of 8 days. The results were presented in the form of a graph; I have read the results from the graph (to the nearest half-unit) and report them below. Length of Grass Blade (units not reported) Not MWed MWed 2 min. MWed 5 min. MWed 10 min. Day 1 1.0 1.0 1.0 1.0 Day 2 3.5 2.5 2.5 1.0 Day 3 7.0 4.3 4.3 4.0 Day 4 10.0 8.0 8.0 7.5 Day 5 13.0 12.0 11.5 11.0 Day 6 15.0 14.0 14.0 13.0 Day 7 18.0 15.0 15.0 14.0 Day 8 20.0 16.5 16.5 15.0 Several facts are apparent from these data. First, the plants that grew the most rapidly were the plants that were watered with water that had not been microwaved at all. Microwaving the water at least two minutes clearly reduced the rate at which the cat-grass grew. Second, the longer the water was microwaved, the stronger the growth-reduction effect on the cat-grass. The plants grown using water that had been microwaved for 10 minutes experienced the greatest slowing of growth, and this was true for Day 2 through Day 8. Third, while the plants were of equal height on Day 1, the effect of the type of water used began to be apparent from Day 2 onward; that is, an effect of microwave irradiation of the water began to be observed immediately. Fourth, the magnitude of the effect was at least roughly proportional to the duration of the microwave treatment of the water. On some days there was no difference between the plants watered with 2-minute and 5-minute microwaved water, but otherwise, there is a steady progression in the effect, according to the duration of the microwave irradiation of the water. Fifth, the use of microwaved water did not kill any plants or leaves; it merely reduced the rate of their growth somewhat. This experiment shows quite convincingly that microwaving the water used to water the cat-grass had a negative effect on the growth of the cat-grass. Of course, this experiment simply demonstrates the existence of a phenomenon; it doesn't explain WHY the phenomenon occurred. One plausible hypothesis is that the structure of the water was changed by being microwaved. If this is what actually occurred, the duration of microwave irradiation appears to determine how much of the water underwent a structural change that apparantly made it less "bio-available" to the plant. The results of this experiment suggest that the correct answer to the question is: "Plants will not grow as well when watered with microwaved water as when watered with water that has not been microwaved, but they will still grow, so long as the water has not been microwaved for too long a time." The second experiment, done in 2006, was the project of a 6th-grade girl. She used two plants (unspecified); each plant was in its own pot and each plant had been previously grown, and then had been cut back to encourage the growth of new shoots or leaves from the existing stump of stem. The two pots were of the same size and type. There were two categories of water used in this experiment. For one, water was boiled on the stove; for the other, water was boiled in a microwave oven. The water in each category was cooled to room temperature before it was used to water the plants. The plants seem to have been watered from above; if a measured amount of water was used, this fact was not reported. The duration of boiling of the water was also not reported, although the earlier experiment made it evident that this was a parameter that had an effect on the outcome. The results of plant growth were documented photographically. The photographs were posted on the Internet at <http://www.execonn.com.sf/> and the results were dramatic, though not immediate: the plant that was watered with the microwaved water had lost all its green foliage by the end of the experiment, in contrast to the healthful-looking plant that was watered with water that had not been microwaved. It seems clear from the results of the results of this experiment that watering plants with microwaved water can kill new plant growth, but it is not at all clear WHY this happened in this experiment. The dramatic results of this experiment are inconsistent in some ways with the results of the earlier experiment. That experiment showed an immediate effect on Day 2 of the experiment, but this experiment showed two apparently healthy plants on Day 2. A clear negative effect took a week to become apparent in this experiment, which is very different from what was observed in the earlier experiment. This experiment has been discussed widely and very criticially on the Internet. Someone asked what kind of container was used to hold the water when it was microwaved. The answer--a plastic container--raises the possibility that some chemical in the plastic was leached out by the microwave irradation, contaminating the microwaved water with a chemical that was toxic to the plant, and that THIS is why the plant watered with microwaved water did so poorly. I find this quite plausible as an explanation for what happened, although a repetition of the experiment with some improvement in the procedure used is the best way to determine for sure what accounts for the results obtained. It would probably take several days for a chemical in the water to be taken up by the plant, and then to interfere with the plant's physiological processes to a degree that would interfere visibly with growth of new tissues. This could explain why it took longer for an adverse effect on the plant watered with microwaved water to become apparent in this experiment, compared to the 2005 experiment. The results of this experiment certainly suggest that it can be damaging to a plant to water it with water that has been previously microwaved, but an altered structure of the microwaved water seems unlikely to explain the results of this experiment. I conclude that different mechanisms produced the damage suffered by the plants in these two experiments, because the way the effects in the two experiments developed over time was quite different, and the observed effects after a week were also very different. The "standard answer"--that microwaved water, after cooling, should be just as healthful for plants as water that has not been microwaved--seems to be incorrect, as judged by the result of these experiments. The results from the 2005 experiment, especially, support the hypothesis that something is intrinsically different about microwaved water in comparison with water that has not been microwaved, and that this may indeed be the structure of the water. Clearly, this is a question that calls for further experimental scientific investigation. Sciencelady

Plants will grow but their roots systems are dwarfed when watered with previously microwaved but cooled water. I know for a fact by observing my own seed experiment that microwaving water does indeed NEGATIVELY effect the outcome of plant growth. The effects were more obvious when the experiment was with larger seeds that need larger root systems (cucumbers) to support the growing plant. The effects were not obvious in radish seeds. Which I believe is because the seeds are smaller and the plant grows faster with a less intensive root system. This hypothesis needs further testing. I grew 8 cucumber seeds in Jiffy, nylon covered peat pots.(The kind that expand when you get them wet the first time) 4 peat pots were watered with water that was heated and boiled on the stove (20 minutes in a stainless steel pot) and 4 peat pots were watered with water that had been microwaved (20 minutes in a glass jar). The water was cooled and saved in separate clean glass jars with lids. As the cucumber plants sprouted and the roots grew through the nylon it became apparent that the root structure was dwarfed in the 4 plants that received the cooled microwaved water. These plants were slightly smaller and did not look as healthy. By day 13 the roots that were showing were very fine like tiny pinworms. The roots were shorter and there were significantly less roots poking through the nylon mesh. The 4 plants that received the cooled stove boiled water were more vibrant looking. The roots by day 13 were significantly thicker, significantly longer and there were significantly more roots poking through the nylon mesh. This is an easy experiment to do at home to see if you get the same results. I am going to plant these baby plants outside and see if there is a continued difference in growth once they are in the ground and no longer receiving microwaved water to see if the damage is long term and permanent. My concern is if this much damage was done to the developing baby plants imagine what type of significant damage we may be experiencing on a cellular level in our bodies by constantly eating and drinking microwaved foods and water. I think it would be interesting to design a test with several small rodents feeding and watering with foods and water that had been microwaved vs. heated by stove. It would have to be an animal with a short life span and one that would not get sick eating heated/ cooked foods such as cooked vegetables. And compare how the animals grow, or fatten, the animal's general health, their hair, and their life spans.