By Meryl
We “borrowed” this gardening advice from an About.com story called "10 Common Container Gardening Mistakes," and then we edited it for urban import. Now, please borrow it from us.
1. Drowning Your Plants: To avoid over-watering your container gardens, use containers that have lots of drainage holes. Also, read (and follow) the moisture requirements for your plants. Before watering, check if the soil is moist by putting your finger into the soil about an inch. Water if dry.
If you do over-water, leaves may turn yellow and fall off, or your plants may get limp. If your soil is too wet, move the 
container to a dry, breezy spot until it dries out.
2. Under-Watering Your Plants: Most container gardens need watering at least once a day in the heat of the summer. Many, especially hanging planters or small containers, need watering more often because there is less soil to hold moisture. When you water, make sure to soak your plants – if you just give them a sip, the water will only wet the top layer of soil. Water until you see it coming out of the bottom.
If your plants dry out, don’t despair. If the container is small enough, submerge the whole thing in a bucket of water until the air bubbles subside. For a large container take a skewer or stick and gently poke holes deep into the soil to allow water to reach the roots. Then water generously.
3. Clumsy Plant-to-Pot Ratio: A large container stuffed with short plants can look stunted. If you need a rule of thumb (and remember that rules are meant to be broken) try to have at least one plant that is as tall as the container. Also try plants that will spill over the sides.
4. Buying Weak or Sickly Plants: Buying plants at a reputable local nursery is a good place to start. You have a greater chance of getting plants that are disease- and pest-free than at a big box store. At a nursery, you can often get a wealth of information and advice from knowledgeable staff. Don't be afraid to ask someone to help you pick out a good plant. See Going Further for listings in your area.
If you can’t resist the prices of buying plants from a big box store, try to buy them on or close to the day they’re delivered. Don’t be shy to ask someone who works there which day new plants arrives. Delivery is usually the same day every week.
5. Fear of Pruning: When your container gardens start looking leggy or ragged, don’t be afraid to cut them back. You may want to put them in an out-of-the-way spot until they rebound, but chances are they’ll come back healthier and happier with a good snip.
6. Overly Mixed-and-Matched Plants: Make sure that all the plants in your container garden share the same sun, soil and water requirements. You can find out this information from your seed packets or plant labels.
7. Starving Your Plants: Most potting mix has very few of the nutrients that plants require to grow, so you will need to add those nutrients to the soil. There are many fertilizers to choose from and flowering plants have different needs than vegetables and herbs.
In container gardening, nutrients in your potting soil are either quickly used by the plants or are washed out with repeated watering. Fertilizing container gardens regularly is a key to their success. Start with a slow-release fertilizer with your potting soil and then add a diluted, liquid fertilizer, such as fish emulsion, every couple of weeks.
8. Living with Yucky Plants: Who isn’t guilty of this? If you’ve tried everything and your container garden still looks dreadful, cut your losses and toss it. If only one plant in the garden is icky, just pull out that plant and replace it.
9. Having Unrealistic Expectations: Before you make your container gardens, evaluate how you live. Do you travel a lot during the summer? If so, either get self-watering containers, enlist some help to keep your plants healthy and alive while you’re gone, or get plants that don't need a lot of water.
10. Gardening According to Someone Else's Personality: Are you casual or formal? Some people like overflowing containers with riotous colors and luxuriant blossoms while others like neat, well-planned, formal containers. Have fun, experiment - do what fits your personality.