While this may sound like an odd first tip, it can be a matter of life and death for your plants. When there isn't a big enough hole or holes for water to get out of your pot, your soil becomes too wet and the roots of your plants can rot which causes the plant to die.
The bad news is that many garden pots that are sold simply don't have enough drainage. You can often increase drainage, by drilling, punching or carving bigger holes. However, sometimes it's just easier to buy a pot that does have enough drainage. I would say that the minimum size for a drainage hole is 1/2 inch in diameter for small or medium sized pots. For larger sized containers, look for at least an inch in diameter.
It is a total myth that adding gravel, pot shards or stones to the bottom of your container garden, you will increase drainage. Some people even say you don't need drainage holes if you put these things in the bottom of your pots. Unless you are a really attentive container gardener, who can water perfectly, or you have a plant that likes wet soil (and there are some that do), you need holes in your pots -- preferably lots of them.
I have found that people (myself included) wildly overestimate how much sun their containers get. While you can find a great plant for almost any amount of light, you have to know how much light your container will get before you choose your plants.
To figure out how much direct light your container will get, place it where you want it and then time how long the sun hits it. You can also use a sun calculator to determine your sunlight.
Most potting soil has no accessible nutrients for your plants, you need to add those. The vast majority of plants will need fertilizer added to your soil, in order to thrive. I mix in a slow release fertilizer into my potting mix. To do this, I either mix up a big batch in a bucket, or fill my pot with potting soil and then mix in the fertilizer. I use an organic potting soil and an organic fertilizer. I use either Bradfield Organics All Purpose or Espoma Plant-tone or Flower-tone.
I then fertilizer every week or two with a liquid fertilizer - usually a fish emulsion, seaweed blend. It smells awful, but really helps to give plants the nutrition they need.
Many people use Miracle-Gro, and while it will feed your plants and help them to grow, if you start using it, you will have to continue because this type of synthetic fertilizer will kill the beneficial organisms that exist in the soil. Organic gardening depends on these organisms so once you've used Miricle-Gro, you will have to stick to it or another synthetic fertilizer.
I'm not big on making lists, but I try to do just that before I go to a nursery. I find that if I don't make a list of plants I want for my containers, I have a tendency to be struck with plant panic - I become a deer in the headlights, paralyzed and completely overwhelmed by the choices so either buy too much, or buy nothing at all because I simply can't make a decision.
I have found one of the best ways to avoid plant panic, or at least minimize it, is to decide what I want before I go and make a list. This list doesn't even have to have the exact plants, though if you are ambitious, you can go online or look through plant catalogs and decide exactly what you want. The list can just have the number of pots and the size and where they are going - so you can get plants that fit the pots and know if you need plants for sun or shade or anything in between.
If possible, it is a great idea to bring either your pot, or a picture of your pot with you. Smart phones are great for this. Most nurseries will have someone there who can help you out with your choices. Also, in most nurseries, the plants are organized and labeled for how much sun they require.
When you are choosing plants for your container make sure that they will play well together. This means that all the plants in one pot should all require the same amount light and moisture. If you combine plants with different needs, some of them will not thrive. So, for example, if you have a plant that requires full sun, you want all the plants you choose for that pot to also require full sun. If you have a plant that likes to dry out between waterings, you don't want to put it in a pot with plants that like it dry.
To find out what a plant requires, either check the plant tag or if there isn't one, ask a salesperson. If all else fails, try to look it up on the internet.
6. Read and Save The Plant Tag
Plant tags are critical. They will tell you how big your plant will get, how much light, water and food it needs and how much care it will need. The tag will also tell you if your plant is annual or perennial and if it's a perennial, what zones it will survive in.
The tag will also tell you about your plant's "habit," which means its shape and how it will grow. This is important when considering your container design and how to arrange your plant combinations. For example if you have have a large pot you might want some plants with "upright habits," to give your design height and then some plants with a "mounding habits" for filling in your design. To finish your pot, you might choose plants with "trailing habits," to drape over the sides of your pot.
Try to buy plants with tags. Sometimes I will use a camera or even my phone to take a picture of a plant and then a close-up of its tag, so I can keep a record of the plant and its required care.
7. Sometimes Plants Die
The more plants you grow, the more plants you will kill. It's that simple. Sometimes even the most expert gardeners will kill plants. It's a fact of gardening life. The trick is to know when to give up on a plant. In a mixed container garden, it makes sense to give up rather early, so your whole container doesn't look ugly.
When a plant starts looking dreadful you have a couple of choices. Depending on the plant, you can cut it back dramatically, and hope for the best. For many plants, this is all it takes, and in a few weeks your plant will come roaring back, happy and beautiful. If I can, I move the shorn plant to an out of the way spot until it is presentable.
You can pull out the unhappy plant and put another plant in its place. Depending on how dead and/or precious the plant is, you can try to rehabilitate by repotting it and baby it until it rebounds or you just can't stand the look of it any longer.
However, if your plant is exhibiting signs of serious disease, take it out immediately, re-pot it and either quarantine or put it in a plastic bag and throw it out.
When you buy seedlings in the spring, or if you grow your own from seed, you must harden off your plants. This is a slow and tedious process, but if you don't do it, the chances of your plants thriving will be significantly reduced.
If you live in a colder climate and plan to over winter your plants, they will also need an adjustment period to get used to less light and drier air.
There are lots of people out there who will tell you to fill up your containers with all manner of junk - from packing peanuts to milk jugs. While doing this will make your container garden significantly lighter, it will also make it harder to maintain because it will dry out faster.
The more potting soil you use, the more water retention you will have which will give you a much great margin for error when it comes to both watering and feeding your plants. I don't know about you, but I need all the margin for error that I can get so I fill my pots with a good quality potting soil.
Here's the honest truth - container gardening is hard work. It takes time and attention, and while it doesn't have to be expensive, it can be. Here's some more truth - there is no such thing as a foolproof plant or gardening system - even if you cover all your bases, some plants with thrive and some will not.
This is both the good news and the bad. All of the uncertainty and hard work makes the whole endeavor exciting, rewarding and endlessly interesting. To have the most fun and to increase your chances of success with any kind of gardening, assess how you live before you dive in. No matter what the answer is, there are container gardens that will work with your lifestyle. You don't like to water? Grow succulents and other drought resistant plants. Don't have a lot of extra cash? Make your containers out of found items or haunt yard sales and second hand stores. You have a formal entrance and want a more tailored container look. Choose large containers in classic shapes and plant them with luxurious and eye catching plants.
Whatever your style, there are plants that can work for you. You just have to do a little research and experimentation to find what works.