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At first glance, a pot just looks like a mini garden bed, right? But those pot walls change everything for your plants. In the ground, a plant is part of a whole system that protects it from wild swings. The earth around it acts like a giant sponge, holding water, keeping things cool, and feeding the roots from a deep, living pantry. In a pot, though, your plant is cut off from all that. It’s stuck in a tiny patch of soil, totally relying on you to supply all its needs.
That is why a tomato that sprawls happily in a bed can sulk and yellow in a container three feet away. It is not the plant; it is the conditions, and once you understand how a pot actually behaves, the fixes are simple.
Watering
A garden bed has water moving in from all directions, so the soil around the roots acts like a sponge, and it can keep refilling. A pot has no such backup. The only water in it is the water you put there. And, it is draining out the bottom and evaporating off the top and the sides at the same time.
If you’re using terracotta pots, this can make things worse. There’s nothing wrong with terracotta pots; they’re actually great for many people. But you have to be aware of how they absorb water, or your plants will really suffer. Terracotta is porous and wicks moisture straight through its walls, which is good for drainage and bad for forgetful waterers.
The smaller a pot is, too, terracotta or not, the faster it will dry out since there is less soil to hold any reserve water. On a hot, windy day, a small container can go from damp to bone dry in a few hours, long before you would think to water it again. Bigger pots hold more water and are more forgiving, so if you tend to forget, consider using a larger pot (but not too large, because if the soil is constantly wet and the roots can’t keep up, they can rot).
What to do: Check the soil in your pots daily throughout the summer rather than watering on a fixed schedule. Push a finger an inch or two down into the soil, and if it is dry at that depth, water until it runs out the bottom.
Drainage
Water doesn’t drain out of a pot like it does in open ground. Instead, it pools at the bottom and creates a soggy layer the roots can’t use, called a perched water table. The shorter and wider your pot, the higher up that wet zone creeps toward the roots.
For years, people were told to put a layer of gravel or broken pottery in the bottom to improve drainage. Unfortunately, this layer of rocks actually does the opposite. The water backs up where the soil meets the gravel, and the saturated zone climbs higher into the root ball than it would have on its own. The roots end up sitting in more water, not less.
What to do: Skip the gravel and fill the pot with potting mix all the way down. Make sure the pot has drainage holes and that nothing is blocking them. If you want better drainage, use a potting mix rather than garden soil, which packs down hard and holds too much water in a pot.
Nutrients
When plants are in the ground, they get nutrients from decaying organic matter, soil organisms, and other readily available sources. A plant can pull from a deep, wide nutrient source for a long time before anything runs low.
A pot is more like a tiny bank account with a low balance. Every time you water, and it drains out the bottom, it takes some nutrients with it. The more you water (and container plants need a lot of watering), the faster the soil runs out of food.
Most potting mixes come with a few weeks of fertilizer built in, and once that is gone, the plant is living on whatever scraps remain. If your plants have pale leaves, slow growth, and weak flowering, it is usually because they ran out of food, not because they're sick.
What to do: Start feeding container plants nutrients about four to six weeks after planting, once the starter fertilizer in the mix is used up. A balanced liquid feed every couple of weeks throughout the growing season works well, or you can mix a slow-release granular fertilizer into the soil at planting and let it release nutrients over the season. Heavy feeders like tomatoes and peppers will want more than light feeders like herbs.
Heat Control
The ground stays remarkably cool a few inches down, even when the air is baking, so roots in a bed are insulated by all that surrounding earth. A pot has no insulation, though. In full sun, the sides of the pot will heat up like the side of a parked car, and the soil inside cooks along with them.
Dark pots and metal containers are the worst at this because they soak up sunlight and pass it straight to the roots. Once the root zone climbs past about 90 degrees Fahrenheit, the roots stop working well, water uptake stalls, and the plant wilts even when the soil is still moist. This often fools people into overwatering a plant that is really just too hot. Small pots heat up fastest, too, for the same reason they dry out fastest.
What to do: Keep container plants out of the harshest afternoon sun in midsummer, or group them together so they shade each other's sides. Light-colored pots stay cooler than dark ones, and thicker walls insulate better than thin plastic. A layer of mulch on top of the soil will hold moisture and shade the surface, just as it would in the ground.