Looking for all kinds of reuse possibilities of plastic (PET) bottles, I came across a list of 48 uses for PET bottles at the site :
Here are some of these uses related to plant growth or gardening:
3. Rain gauge.
An inverted bottle with the base cut off. A neck section can be inserted, as a funnel, to reduce evaporation. Attached with loop/strap – see #2.
4. Wall-mounted pot plant holder.
An inverted bottle with the base cut off and a hole in the cap or no cap. Several in a vertical row could use the drips from the one above. Attached with loop/strap – see #2.
I first saw this in 1980, when PET bottles were quite new to Australia.
22. Mini compost heap.
Cut the neck from the bottle and turn it upside down, stick it over small plants. This is called a “cloche” in Canada.
41. Flying insect traps.
Randy Crawford uses these to catch insects for his frogs. He uses two types of traps and has described them as follows: Trap 1 is a bottle with the top 1/3 cut off, trimed so it makes 3 legs (holes for the flies to access the bait which is put under this affair) and inserted into the other 2/3. After the flies feed they always leave by taking off and flying upwards, thru the funnel and into the main chamber. They are too stupid to try to get out the hole, always trying to fly up. Yes, it really works.
Trap 2 has short pieces of tube inserted in the sides, pointing down. The bait is put in the bottom, the flies enter thru the tubes, and again are too dumb to get out that way. They actually sell a trap in Canada very similar in design (no 2 litre bottle, though).
43. Slow release watering system.
In Australia in the mid 1980s there was sold a screw on spike with holes in it. The idea was to have a water filled bottle and stick the spike into the ground near a young seedling so the water was released gradually. Probably a small hole to admit air in the base of the bottle would help.