KC Gardens

Plant annual flowers by soil temperatures

Soil thermometer

Tip of the Week

Plant annual flowers by soil temperature

OLATHE, Kans. — What a wild and crazy year it has been, especially if you compare it to last year. Last year the season was ahead by about a month. This year we are behind average by a couple of weeks. Dr. Alan Stevens, our K-State Extension specialist and lead for the Prairie Star Annual Flowers research, offers some very good advice. Unfortunately it may come too late, as many of us have probably rushed to plant the annuals.

We often plant our annual flowers based on the calendar, without giving any thought to the soil temperatures and the effect it has on root development. Read Alan’s comments below to learn more about the importance of warm soils for proper growth and development. The time to plant annual flowers is based on soil temperatures and not the calendar. Some cultivars tolerate cool soil, while others require warm soil. This spring we have experienced such wide temperature fluctuations that it is difficult to know what to do.

This from Dr. Stevens: An easy way to gauge the soil temperature is to simply stick a thermometer about 4” into the ground. You can take a measurement in the early morning and late afternoon to get a high and low for the day, then average them. K-State also has a good weather data library that shows soil temperatures at weather stations throughout the state. Currently, our soil temperatures are still in the low 60’s, which means it is too early to plant based on the fact that most annuals that thrive in the summer like the warmer soils. For more information on the soil temperatures in the state visit: http://www.ksre.ksu.edu/wdl/Text%20files/text/REPORT.TXT. Unfortunately, the closest reading for the Kansas City area is Ottawa, Kansas, just south and west of the area.

When the soil temperature is around 65 degrees, petunias, begonias, alyssum and snapdragons can go in the ground. More sensitive crops like vinca, celosia, lantana, melampodium, zinnias, and pentas need soil temperatures of 68 – 70 degrees. These are the flowers that thrive in the heat of the summer and need those high temperatures. Most everything else falls in between.

This is the year to be patient. Wait until soil temperatures are up for a couple of days before rushing out to plant. Our most heat tolerant plants tolerate the heat because they love and thrive in warm soils. Cool, damp soils will comprise their root systems and then, when the real heat of the summer does finally arrive, they will have few roots to support the plants growth. Happy planting!

Dennis here: Thanks, Alan, for this gentle reminder that, as gardeners, we sometimes get in a rush to plant and don’t stop to think what the plants prefer.


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