So you're taking off on a vacation? Perhaps even a green ecotourism trip? Good for you! Studies have shown vacations are good for your health. But now you have to figure out what to do about watering your plants. The good news is that it's easier (and cheaper) to leave plants to their own devices than it is pets. It just takes a little planning and forethought.
Well-watered houseplants will last for days to perhaps a week on their own. If you're heading out for only a few days, give them a final drink just before you leave and move them out of sunny windows or hot rooms. Outdoor potted plants will dry out faster, so give them a good soaking before moving them into a cool garage or laundry room to slow down their water use.
For longer vacations, however, you'll want to do more.
Find an Educated Neighbor
If you have a plant-savvy friend who can come over a couple of times a week while you are away, in exchange for your doing the same in return, that's ideal. Even a careful non-plant-savvy person will work in a pinch if you do a little pretravel planning. For a few weeks before you leave, keep track of how much water each plant tends to need and how often, and then leave very specific instructions ("give this plant ½ cup of water every weekend"). Help your friend out by grouping plants with similar watering needs together on a waterproof floor and out of direct sun. Remember, your house may be warmer while you are away in the summer, so adjust care instructions to account for faster water use.
If you have a manageable number of plants, you can make a water-recycling terrarium out of a clear plastic bag that will keep them happy for months. Put the open bag on a waterproof floor in a room that will stay at a moderate temperature (cool in summer, warm in winter) and out of direct sun. To avoid tearing the bag, carefully spread a moist towel along the bottom, and arrange as many well-watered potted plants on the towel as will fit. Pull up the sides of the bag over the plant(s), blowing in air to puff out the bag, and twist it shut on top (you can seal it with a twist tie or a rubber band). For an extra-airtight seal, fold the twisted portion in half, and wrap it with another rubber band. The plants inside will release water from their leaves, and the excess will drip back down onto the leaves and potting soil, where it will be available to the roots again. I used the same plastic bag for three years to get my plants in my dorm at college through school-year holidays, and they were still growing when I graduated. You can do the same for plants in outdoor containers. But whether your plants are indoors or outdoors, move them out of direct sunlight, which will turn the bag into a solar cooker.
Wick-ed Easy Alternative
If you have large and unmovable plants, you prefer to avoid plastic bags, or you just have too many to move, you can set up an easy wicking system that works indoors or out. You need some sort of absorbent wicking material, such as thick yarn, scraps of old natural-fiber rope, or strips of a cotton T-shirt (set up a test pot to make sure your wick wicks well), and containers to hold water, like bottles, bowls, or pails.
Set a container of water next to the plant; a single container can serve multiple pots if it's large enough. Place one end of the wick into the water, making sure it reaches the bottom of the container so your plant won't be left high and dry as it drinks, and poke the other end about three inches deep into the plant's moist soil. As the soil dries out, water will travel up the wick to replenish the moisture.
If you have lots of outdoor containers, you may want to consider a drip watering system with an automatic timer, which will not only take care of them while you are away, but will also save time and make it easy to keep up with them even when you are home. These systems are very simple to put together and require no special tools (other than a punch that will come along with the kit) and you can arrange them to fit your garden's layout or containers' needs so there'll be no water where you don't want it! A basic system will set you back about $100; it will last for years, and you can even redesign it as your plans change.