Question
For several years I set out periwinkle, a different color each year, in my front flower bed, and had beautiful flowers year after year.  Finally, one year, the flowers did not thrive as before and looked seared and eventually died.  When I pulled the plants up the roots had rotted away.  Since then I have set out several different bedding plants.  The only thing that has thrived in the bed is liriope.  A back bed where I put tomatoes for several years has done the same thing though Indian hawthorn has thrived.  Do you know what the problem is and what can be done to solve it?

Answer
Hi LaDelle, You may have more than one problem.  There is a fungal root disease that attacks periwinkle and it's difficult to control.  I usually recommend a soil fungicide and not replanting periwinkle in the same bed for 3 years.  But since it's affecting all your herbaceous plants and not your shrubs, I suspect another problem.  Often over the years, a layer of decomposed organic matter forms on the soil surface from mulch and leaves.  It looks rich and black but in fact, not much will grow in it. It doesn't hold water or nutrients well and plant roots take the path of least resistance and don't penetrate the soil layer beneath. It's very important when planting to dig deep enough to get to the soil.  Loosen it, mix some of that surface organic matter with it and make sure that the roots of the flowers or vegetable are in the soil, not just the loose surface layer.
Use a good quality, timed release or organic fertilizer when planting and spray feed your plants once every couple of weeks for a little added boost of nitrogen.  Any water soluble fertilizer will do. I think this will fix the problem.  
If the problem continues, take one of the plants, roots and all, and some of your soil to a local garden center.  They should quickly be able to tell you whether it is a disease or just a cultural issue.  Jim