Choosing the right perennials for your garden requires few criteria, which you need to follow so to provide healthy growth of your plants. Before buying the perennials, you should consider the soil of your garden.
Some perennials require specific soil, others grow with special fertilizers. In case you have moss and germinating weed seeds, you need to consider your soil needs an improvement before growing perennials. Use fertilizers, which are enriching your soil. 
Insects and diseases are the most common issue, when it comes to perennials.
Be aware that the plants with leaf spots can grow fungi and other disease, which can easily be transferred to the perennials, so before planting them, always check your garden and remove those plants, which are already infected.
The healthy perennial has a multi-stemmed crown and its roots are reaching the bottom of the container. Actually the healthy and well-grown perennials have roots just as the potatoes.
