What should I use to water my plant when I’m away?

I have a few lemon trees in my house that I started from seeds. They’re a couple of years old and about two feet high.

They are surprisingly very little work. The longest I have left them alone is 5 days, and when I come home they look great, no droopy leaves. Luckily, I have not forgotten to water them right before I travel.

If I left for more than 5 days I’m afraid it might put a strain on the plants from lack of water.

From what I’ve read, I need to be really careful not to overwater and especially not to let the soil stay wet for an extended period. Typically, I water the plants 2–3 times a week.

What are your best suggestions for watering when I am away?

My lemon trees don’t seem to do very well outside.