Become a Smarter Gardener in 2019: Fertilizing and Watering Container Plants

Photo: Julie Weisenhorn, UMN Extension

Container plants are a great way to have lots of green in a small space. They often require a bit more attention than plants growing in a garden, but a little maintenance can go a long way towards healthy, productive container plants.

Fertilize regularly 

Even if you used a potting mix with a slow-release fertilizer, repeated watering will leach nutrients over time. It's a good idea to start regular fertilizer applications anywhere between 2-6 weeks after planting a container, depending on the type of potting media used, watering schedule, and rate of plant growth. 

Example of all-purpose plant food.  Photo: Anne Sawyer, UMN Extension

There are lots of options for fertilizers to use in container plants. A good place to start is with an all-purpose fertilizer. All-purpose fertilizers have nitrogen, potassium, and phosphorus - the nutrients that plants need in large amounts - as well as other essential plant nutrients like iron, manganese, and zinc.

To promote flower or fruit (rather than leaf) production, you can also select fertilizers with higher amounts of phosphorus or potassium relative to nitrogen, such as a tomato food or bloom boosting fertilizer.
 

What's the best--soluble or slow-release fertilizer?

A soluble fertilizer is one that will easily dissolve in water, allowing plants to access the nutrients right away. Soluble fertilizers are easy to use and are a good choice for container plants, where rooting space is at a premium and nutrients are often lost through frequent watering. 

When nutrients are needed quickly - such as when the lower leaves of your tomato plants are turning yellow because they don't have enough nitrogen - a soluble fertilizer is the best choice for plant health. Slow-release fertilizers can also be used effectively in containers, where watering releases small amounts nutrients over time. 

Large, fast-growing plants may need more nutrients than slow-release fertilizers can provide, so keep an eye on your plants and supplement if needed.

Over-fertilized pepper plant.  Be sure to always follow label (or soil test)  recommendations when fertilizing!  Photo: Anne Sawyer, UMN Extension

Regardless of which fertilizer you select, it's important that you follow the label to avoid over-fertilizing, which can damage plants and send excess fertilizer into the environment.

You may, however, want to consider more frequent fertilizer applications at a lower rate to prevent nutrient loss with water drainage. For example, if a fertilizer calls for one "scoop" per gallon and you fertilize once every two weeks, try using a half-scoop per gallon and fertilize every week instead.

Maintain even moisture

Maintaining even moisture - usually by applying approximately 1" of water per week for in-ground gardens - is challenging for container plants.

Depending upon the size and material of the container, you may need to water more than once per day during hot, dry weather. At a minimum, you'll usually need to water at least daily. Avoid letting your containers dry out for too long - plants can become quite stressed from prolonged periods without water.

Mulched pepper plants in containers.  Photo: Anne Sawyer, UMN Extension

Mulch is good for containers, too!

This year, however, I finally learned to garden smarter with my container plants - I mulched them all.  I'm absolutely amazed at how much the mulch has cut down on my watering.

Not only does mulch minimize water loss from evaporation, but it also moderates soil surface temperatures, keeping plant roots a bit cooler in the hot summer sun. Furthermore, mulch prevents soil from splashing onto plant leaves during rain or watering. Splash can spread plant pathogens from the soil to plant leaves, which can be particularly problematic for tomatoes, among other plants. 

Just as mulch helps keep weeds from germinating in containers and raised beds, it may moderate desirable plant spread, particularly if you mulch too closely to the plants. Mulch may not be the best fit for all containers, but keep it in mind as a maintenance- and water-saving tip!
 

Small mulched raised bed with flowers.  Photo: Anne Sawyer

For my container plant mulch, I used grass clippings from my herbicide-free lawn. If you don't have access to grass clippings, you can try a natural fabric like burlap or even a bit of straw or hay, if your containers are large enough.

Shredded newspaper, coconut coir, or other natural materials may also work as mulch.

Watch out for soggy soil!

Finally, if you use trays to catch water (and nutrients) under your plants, be sure that the retained water does not cause prolonged periods of waterlogged soil.
 

Container basil showing nitrogen deficiency (yellowing of lower leaves). Water was not routinely emptied from the tray underneath.  Photo: Anne Sawyer, UMN Extension

Most container plants prefer moist - not soggy - soil. Saturated soil can also lead to nitrogen loss from a natural process called denitrification, where bacteria convert a plant-usable form of nitrogen (nitrate) into gaseous forms that move from your soil to the atmosphere. In other words, be sure to dump your water-catching trays regularly to prevent waterlogged soils and nitrogen loss.
 

Author: Anne Sawyer, PhD, Extension Educator in On-Farm Food Safety

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