Our research showed that plants grow better with lower fertilizer concentrations at higher temperatures. Other environmental factors may affect optimal fertilizer concentrations as well. It all depends on how they affect water use and growth of plants.

When plants use a lot of water but grow slowly, they benefit from lower fertilizer concentrations than when plants grow fast, but use fairly little water. Here is how different environmental conditions affect optimal fertilizer concentrations.

Light level. Both the growth and water use of plants increase with increasing light levels. These effects tend to cancel each other out and optimal fertilizer concentrations are probably not affected much.

Temperature. Because plants use more water at higher temperatures, plants will benefit from more dilute fertilizer rates under warmer conditions.

Humidity. Plants use less water when humidity is high, but humidity has little effect on growth of the plants. Therefore, plants will need higher fertilizer concentrations when humidity is high.

Wind speed and air movement. Air movement generally has little effect on plant growth, but it increases the amount of water the plants use. When there is a lot of air movement, fertilizer concentrations should be lower than when there is no air movement.

Monitoring EC. Light, temperature, humidity and wind can affect optimal fertilizer concentrations. Unfortunately, it is difficult to predict exactly what the optimal fertilizer program is in a specific greenhouse. However, by monitoring growing medium EC you can determine whether there is enough fertilizer in the medium for your plants. For petunias and many other bedding plants, we recommend a PourThru EC of 2-3 mS/cm.