You’re going on a short holiday, but while you’re away London’s going to have the hottest day of the year and you don’t want to torture your plants. One thing you can do is set up a gravity-feed drip watering system using drip irrigation spikes. But they kind of suck: when there is a lot of water in the system, they drip quickly, and when there is hardly any water, they don’t drip at all. You basically can’t set the drip rate you want because it will change almost immediately.
There isn’t a constant drip rate because there isn’t a constant amount of water, so the solution is to provide one. You can do this by using two bottles, one of which continually feeds the other to maintain a given level. Here’s how.
First, take your first bottle and cut it.
Then, add the watering spike.
Now, take your second bottle and make a hole near the top. This bottle is going to be inserted upside down in the second bottle, and the hold should be below the surface when that happens.
Now, cover the hole with your finger and fill the second bottle with water. Then invert it and place it in the first bottle.
Note that there should be an air gap between the first and second bottles, i.e. there should be no seal where the bottles meet. If you do this right, the second bottle will empty into the first bottle until the hole you made is covered by water. After that point a partial vacuum will form at the top of the second bottle and it won’t empty any further. You can now adjust the drip rate, which will be based on the level of water in the first bottle.
When enough water has dripped out from the first bottle, the hole will be uncovered, allowing air to enter the second bottle. This will allow some more water to drain from the second bottle to the first, again covering the hole.
This set-up seems very precarious, but it worked for my (admittedly only five-day) holiday. Here is the contraption applied to a tomato plant: