Slide any nursery-grown plant out of its 1-gallon pot and cut the rootball into two equal halves. You might be surprised to find that the roots are clustered around the outer edge of a largely untapped soil mass. This is because plant roots go to where there's water. And water applied to container-grown plants tends to travel down the seam between the soil and the inner pot edge.
This yields an important lesson: Most plants are utilizing only a small fraction of their potting soil. If you can encourage them to root into the center of the soil mass, you can increase the plant's vigor without transplanting. The more roots there are, the more resilient your plant will be in heat and drought. It also will grow faster and flower better.
The trick to encouraging more adventurous rooting is to change the way you water. The goal is to evenly wet the entire soil mass, not just the outer edge. This takes time, particularly with a very dry rootball that resists water absorption. But once saturated, you need not water as often.
The simplest way to completely saturate large, stationary pots is to turn the garden hose on the barest trickle or drip and place it at the center of the pot, which may be right next to the trunk of a plant. Water slowly applied here will be drawn down by gravity directly into the center of the root ball. After a few hours it will be well saturated.
To water portable pots I use a rectangular plastic box designed for under-bed storage about 6 to 12 inches deep. I put my pots into the box and fill it half way with water. Then I water each pot until it won't drain. That means pressures have equalized holding water around the root ball instead of letting it flow through and out the drain hole.
I let them sit in the water for an hour to be sure the entire soil mass is saturated. Then I remove them to drain outside or in the bathtub. In go the next round of pots and the process repeats itself. Sometimes, I mix fertilizer into the water for an extra boost.
You can use the same method with hanging moss baskets that can be tough to water evenly. Use a large plastic garbage can with a smaller one turned upside down inside it. Fill with water and drop your hanging baskets in to sit on the upturned can for an hour to really saturate the sphagnum moss and soil. Then, remove the basket and hang to drain before returning it to its permanent location.
If you find your pots aren't draining well, they may have a very small hole that has become blocked, or the pot may be too flat on the bottom so that when it is placed on concrete or other nonporous surfaces there's barely any gap between the pot bottom and the paving. This can inhibit water flow considerably.
To enhance drainage, you need to get your pots up off the ground, if just for a 1/4 inch. There's no need to invest in expensive pot feet. My preference is to use small, ceramic tiles or fragments of larger ones. I simply slide them under the edges of the pot where they can't be seen to raise it up. This gap can make a real difference if you are growing drainage-sensitive plants or if you tend to water too much.
Another trick is to be really careful about what kind of potting soil you use. Some brands have way too much woody matter in the mix. You'll see too many of these wood chips in the mix and not enough fine humus. Wood can result in fungus growth and can cause nitrogen deficiency. Select your soil with care. Mix in a 2-to-1 ratio of coarse sand and perlite to give it improved drainage. Fertilize often to ensure your heavy-feeding annuals are fat and happy.
The most common reasons that container gardens and potted plants do poorly or fail altogether are over-watering or drainage problems. So, before you start anew this year, make a change to ensure every inch of potting soil becomes a home for adventurous roots.