I have used the following technique:

1. Start a couple of days before you leave.
2. Get an empty 2 litre bottle with that 5-lobed moulding on the bottom
as used for soft drinks, beer, etc.
3. Put seedlings, plants etc in a tray but leave enough space for the
bottle upright.
4. Put about 1/2" of water in the tray.
5. Cut a hole about 1/2" in diameter very close to the bottom of the
bottle.
6. Block the hole with a finger, fill the bottle with water and replace
the cap
7. Place the bottle upright in the tray and take your finger away.
8. A little water should trickle out and some air bubbles go in.
9. A day or two later, check that the level in the bottle has fallen
slightly.
10. Any trouble with steps 8 & 9, enlarge the hole.

Consider using several bottles if the weather may be hot or the holiday
extended.

Steve Harris - Cheltenham - Real address steve AT netservs DOT com
A useful bit of gardening software at http://www.netservs.com/garden/