I have a little problem with this one. Why would God answer a prayer to make a plant grow faster for the sole purpose of proving that praying to him can make a plant grow faster? I can see why he might answer such a prayer when the object of the exercise is to get enough food to keep a family from starvation, but not for a daft reason like this.
God can choose whether to answer a prayer or not, so if it doesn't work what conclusions can you draw?
1. Prayer doesn't work because there is no God
2. God chose not to answer this particular prayer
So a negative result means that God either does or does not exist. Seems to me we're no further forward than we were at the start. If it works and the prayed-over plants grow faster is it because of the increased carbon dioxide breathed over them? How on earth are you going to control this experiment? You'll need to separate your plants because you also have the possibility that talking softly to plants makes them grow faster and being nasty to them or ignoring them makes them grow more slowly. Once they're separated how do you know that they are subjected to identical growing conditions eg amount of light, temperature, water, the starting condition of the seed, presence of plant pathogens.....the list is endless. I'm not saying it couldn't be done, but the difficulties mean that you have to have access to specialised equipment to be sure that prayer and only prayer is the only variable in the experiment.
Also, isn't there a quote from Jesus during his 40 days in the wilderness when Satan tempted him to jump off a pinnacle and the reply was along the lines of "Do not put the Lord your God to the test"? Just a small point.
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