Jenny wrote:

Hi what would be the best glue to use to glue plastic tubing to small
plastic pots that would be full of water. I was going to use silicone
sealant but it says not to use in permanently damp conditions. The joint
would in water all the time. This is to make a watering system for a
greenhouse.

Umm why would you want to glue the tube to the plantpots? It's a bizarre
way to set up watering. The normal way of doing this is to use a "common
rail" tube running along the greenhouse bench with "T" pieces at regular
intervals. Fron each "T" you take another length of tube at the end of
which is a drip controller (a small valve, operated by adjusting a
thumbscrew to set the drip rate) which you hang into the top of the pot.

Then you can move the pots if necessary, adjust the water flow
individually and have a neat system that doesn't need mucking about
with.

This illustrates the system used with growbags, it's similar for use
with pots:

http://www.twowests.co.uk/public/pic...rge/SDBDWK.jpg

Here's a link to the tube, valves, reservoirs and materials. The costs
are so low that it would probably cost you more to create a homebrew
version:

http://tinyurl.com/4afoay

And here's an American site that explains the principles:

http://www.dripirrigation.ca/introduction.asp