Timothy Murphy wrote:
I'm going away for a month,
and am wondering if there is a simple way
of watering window-boxes while I am away.
I have a number of 500ml empty sparkling water bottles.
I tried making a hole in the top of one,
but it is very difficult to control the amount of water
that comes out.

There are commercial gizmos to do it. How well they work is a mute point eg.
http://www.travena.co.uk/watering.htm

I'm wondering if one could use a piece of string
with one end in the bottle and one in the soil,
with osmosis slowly taking the water from the bottle?

It would be capilliary action that does it. As you describe it would
lose too much by evaporation, but you might be able to get it to work by
sleeving the wick with cling film.

There is a moderately cute way to do it with a small amount of metal
work and a bit of ingenuity. Based on expansion of air when the sun
shines. You need some soft brass or copper capiliary pipe and bend it
carefully to form a J shape. Then drill a matching tight fit hole
through the centre of the drinks bottle cap. The long leg of the pipe
goes down into the water and the J drips onto the plant. You may need to
attach some black PVC tape to the part above the air space to get the
right amount of water to come out. It is the air expanding that make sit
work. At night when it is cool air is sucked back into the bottle and
the cycle starts again. The amount of water delivered depends on how
warm it is during the daytime. More water delivered as the air space
increases.

You need a gas tight seal on the hole for it to work - a spot of impact
adhesive or bath sealant will do. You can buy gizmoes to do it too but
they are expensive for what they are and fail fairly quickly.

Make sure there is a diffuser around the bottle. The sun through a clear
cylindrical lens at this time of year can start fires!

It always pays to test irrigation systems for a few weeks before you go
away. I came back once to find a bunch of rare desert plants in my
greenhouse under 2" of water - amazingly they tolerated it.

Regards,
Martin Brown