If you want outstanding container flower gardens or abundant, tasty vegetables in pots, you have to both water and fertilize them. And the latter can present a problem.
Container plants dry out much more quickly than in-ground plants, a fact that is readily visible, as the leaves wilt quite dramatically, and the need to water more frequently is well understood by most gardeners, who therefore water them more frequently. Check! But what gardeners often fail to understand is that plants grown in containers (flower boxes, hanging baskets, planters, etc.) also need more minerals (fertilizer) than plants growing in the ground. That’s because the minerals are regularly leached out by rainfall and watering.
What is Leaching?
When rain falls on a plant growing in the ground (or when you water it), minerals present in the soil tend to percolate downward as the water does. However, when the rain stops and the soil nearer to the surface starts to dry out, water is “pulled” back up from below due to capillary action … and minerals follow. The result is that minerals tend to stay more or less within the plant’s root zone. Perfect!
When you water a plant growing in a pot or it rains, excess water drains out of the pot … and so do its minerals. They can’t move back up as the soil dries out. And since you water container plants much more often than garden plants, often several times a week, this repeated leaching impoverishes the soil that the container plants grow in, leading to weak, lackluster growth and decreased flowering and yields.
The Two Fertilizer Solution
Many gardeners have found that the easiest way of keeping their container plants productive is to combine two fertilizer strategies.
First, mix in a slow-release fertilizer (usually good for at least three months use) into the potting medium at planting time in spring, according to the label (and also mycorrhizal fungi, very efficient at helping plant roots collect both minerals and water). Fertilizer spikes and tablets are also slow-release. If you didn’t do so in spring, you can at any time during the summer (by fall, though, it’s probably too late for slow-release fertilizers to be of much use, at least in climates where plants stop growing for the winter).
Then also fertilize with a soluble fertilizer, adding it to your watering can and applying each time you water your plants*. This is called the constant fertilization method.
*During periods of extreme heat, it may be wise to stop fertilizing for a while, especially if your plants react by ceasing to grow. You can restart your regular fertilizing method when temperatures return to normal and plant growth is relaunched.
If you water using a hose, you’ll find a variety of hose-end sprayers to which you can add soluble fertilizer.
Most soluble fertilizers are labeled for use once per month, but it’s easier to remember when it’s time to fertilize if you add them at each watering: then it becomes part of your routine. However, it’s very important to dilute the fertilizer accordingly: typically, you’d apply them at one quarter to one eighth of the recommended monthly dose. And the dose, of course, will vary widely, according to the brand of fertilizer you use. (Hint: read the label!)
Organic or Synthetic: You Choose
Note that there are both organic and synthetic slow-release fertilizers and organic and synthetic soluble fertilizers. Personally, I prefer the organic option and use a slow-release organic one, such as chicken manure, early in the season, then complete with liquid seaweed fertilizer throughout the season.
Whatever method you choose, do keep in mind that container plants will need more minerals than your garden plants … and react accordingly!