Answer: Most perennials decline in vigor over time and need to be divided routinely. Three to maybe five years is the usual time frame for division.
Also, over time the original soil amendments become depleted. Fertilizer alone is not enough to replenish and feed the soil in the long term. It is important to provide regular topdressings with good quality compost and to use an organic mulch year round; these break down and feed the soil on an ongoing basis.
Finally, weather also plays a role. Drought or an excessively long cold winter or severe late frosts after growth has begun can set plants back for the entire season.
I hope this helps you trouble shoot.
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