I'm reading about burying most (all) of an upside down plastic bottle next to a plant so you can get water down to the roots... but doesn't it also allow hot dry air down next to the roots? Wouldn't it actually AID evaporation by getting the side of the plant warmer than the mulched top?
I'm thinking buying/driving a 1.5-2" piece of PVC and leaving the end stuck in the air might be better, unattractive to look at,but would create a skinnier hole and therefore less heat down at root level while maintaining the idea of a resevoir of water to feed down into the lower roots...?
I don't know about the hot air getting to the roots, but this year I buried plastic coffee cans with holes poked in bottom and one side next to my tomato plants and it's working great. I usually have a problem with leaf spot and other fungal diseases and so far this year I don't even have a yellow leaf and the tomatos are looking great.
I don't think heat at the roots will be an issue at all. It is an excellent means of water and nutrients being delivered deep in the root system. It will stimulate deep root growth rather than the shallow surface roots that develop when surface watered.
This is an old style of growing in the southwest. They used porous clay jugs called ollas which were buried in proximity to the plants needing moisture.
If you are concerned, you could always put a lid on it and cover it with mulch to hide the uglies. Just a thought. Kristi