Answer: Unfortunately, the only time you can seriously amend the soil is prior to planting and it is a huge job to lift everything and redo it.
Ideally, you would run soil tests and add amendments based on the results. A perennial garden would be "double dug" or at least tilled down to about ten inches with all amendments such as organic matter or sand thoroughly mixed in at this time.
Individual plants can be planted with additional amendments added to the planting hole to reflect the perennial's particular soil preference. This also allows you the opportunity to add organic matter over time.
This type of planting and preparation is followed up with annual top dressings of compost and the use of a natural mulching material year round, these will help feed the soil on a continuing basis as they rot down. It is very important to keep adding this organic matter since it does break down and need to be replaced.
Sand, on the other hand, does not usually need to be added on a regular basis because it does not decay -- and adding too much sand can result in problems, too.
In many cses it is far more preferable to continue improving the native soil rather than bring in topsoil. The imported soil often is a poor match with the existing soil and this imbalance can create even more problems later. The "topsoil" offered for sale is very often of varying qualities and types as well.
If your soil is truly problematic, you might consider making raised beds for your plantings since this can allow you to improve the native soil in an intensive manner. If drainage is a problem, sometimes a simple increase of just a few inches can make enough difference.
At this point, I would suggest that you consult with your county extension about your soil. At the very least you should run some basic soil tests and chart your amendments based on the results.
I am also somewhat concerned about "blackening" peonies, as this could be a disease rather than strictly a soil structure-based problem -- although it could be a problem that lives on in soil. You might want to consult with them specifically about that as well.
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