Also, the sheer number of plants we own made commercial solutions too expensive. I considered building a system with tubing, but my husband worried that it might spring a leak while no one was home.
So I came up with the following dirt-cheap, easy, and fairly leak-proof solution: You just need resealable plastic bag full of water with a wick in it to pull the water out at steady rate.
This Instructable will show you how to do that, but all you really need to know is that the size of the needle you use to thread the wick determines how fast the water drains out of the bag.
Caveat: Not all plants will be happy with this method of watering, which keeps the soil continuously damp. Ivy, for example, likes to dry out before being re-watered.
Step 1: What you'll need:
1) Thread. Most types will work, but cotton thread is probably best for wicking purposes. If you're going away for a very long time, polyester might be better to ensure the thread doesn't rot.
2) Scissors for snipping thread.
3) Re-sealable plastic bags. The size depends on how large a reservoir of water you need, which will depend on how much water you want to deliver to the plant every day, and for how many days.
4) A very fine needle. The ideal is probably a #10 needle, which is about 0.5mm = 0.02" thick. However, any needle in the #8 - #12 range should work fine.
NOTE: A "regular" needle is usually about a #6 and TOO LARGE. The size of the needle is what determines how fast the water will drain, so you must find a very thin one. It's easy to add more threads to one bag to increase the drainage rate, or to add several small bags to one plant's pot, but you can't throttle the water flow from a hole that is too large.