Question
I am in North Dakota and planted petunias in my flower bed. I fertilized with a slow release 10-16-10 fertilizer at the time of planting. All of the plants are blooming, but the leaves of some are becoming quite yellow.
Thank you for any suggestions.

Answer
Susan,
Yellowing leaves on petunias can be any of the following situations:
1. Too much water.  Most annuals want a deep soaking less often...and although you can't control the rain, you can be sure to water deeply every four or five days instead of every day.
2. Or the opposit - too dry. If the plant dries up periodically, it will shed those leaves that it hasn't had the moisture to support - these turn yellow and fall off.
3. If it's just the lower, older leaves, and the newer ones look fine, this is just the natural way the plant has of growing.  Be sure that the moisture is consistent - if the soil feels moist don't water, and if it starts to look dry on top, feel down an inch or so and if the soil is dry, give the plant a good deep soaking.
4. Too much shade - petunias want to be growing in at least 5 hours of dead-on sun that include the noon hour....if they aren't getting this much, they may yellow and drop leaves.

Your fertilizer is fine - you might need to give them a liquid fertilizer once in the beginning of August to supplement the time-release product at that time because the time-release tends to release the nutrients faster in hot weather.

If you have Wave or Superpetunias you don't have to deadhead them, but traditional types need to be deadheaded by cutting the small stem below the spent flower in order to remove the developing seeds.

All petunias stay fuller and bushier if every week you cut two or three stems back by half starting the third week in June in most places - those stems you cut will branch out so by doing that every week you will keep them fuller and more flower filled all summer.

I hope this helps!
C.L.
www.gardenlady.com