How Much Fertilizer to Use for Annual and Perennial Flowers in Large Containers?

Asked May 27, 2019, 6:01 PM EDT

I have planted sun-loving annual and perennial flowers (outdoors) in two large wooden containers: One is 8' x 2' with a depth of 10 inches. The other is 6 feet long, with the other dimensions being the same.

Here's my question: how much fertilizer do I apply to these containers? Most instructions for fertilizing ask if I am growing outdoors in a garden bed or indoors in a flowerpot. The differences in the recommended amounts are huge. For example, the Miracle Gro website says:
"(for outdoor plants) Mix 1 tablespoon of Miracle-Gro® for every gallon of water. ...
For indoor plants, mix 1/2 teaspoon (not tablespoon) per gallon of water. "
My large planter boxes seem to present a cross between the two aforementioned garden situations.

Other growing info:
I am using organic potting soil, and I put 1-2 inches of permanent "packing peanuts" on the bottom (for drainage) and a moisture mat between the packing peanuts and the soil. The plants seem to do well, and the perennials wintered over successfully with the help of some mulch and insulating material.

Spacing: I allow about half the space between plants that I normally would use if I were planting in the ground.
I also am growing flowers in several smaller containers.

Thank you in advance for your advice. I wouldn't be surprised if many other gardeners are as confused as I am!

Best regards,

Janice Bakker

Denver County Colorado

3 Responses

You can use a water-soluble fertilizer at half strength each time you water or at full strength every couple weeks. You want to provide enough fertilizer to grow healthy plants but not so much that they get long and leggy. Watch how them do and adjust accordingly.

Here's some advice from CSU Extension on growing in containers

Hello again. Thank you for your prompt response, but I am still confused.

Let's go back to my example of the Miracle Gro soluble fertilizer. When talk about using it at "full strength," are you advising 1 TB per gallon or 1/2 tsp per gallon?

You want to follow the label instructions for outdoor plants, and in the Miracle Gro example cited in your initial email, that would be 1 tablespoon per gallon for full strength or 1/2 tablespoon per gallon for half strength.