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Most gardeners understand that making a new garden is easier than renovating an old one. Sometimes the answer is to just pull out everything and start fresh.....this is especially true if grass has overtaken the garden or Junipers planted 20 years ago are looking tired. Grass is probably one of the worst weeds to get rid of. Sometimes one perennial that looked nice in the garden centre is trying to take over the world, and needs editing. This editing can be done any time of year. Don’t feel you have to plant it elsewhere, if it is a thug then keeping only part of it and composting the rest is a good plan. Sometimes however there are plants we really want to keep in place, shrubs we have finally established or specimen evergreens we paid $$$$ for. The answer then is to renovate around the keeper plants. Fall is an excellent time to undertake this work.
Most perennials, shrubs and evergreens respond well to fall transplanting. The exceptions would be any plant in full flower. Peonies are best moved in fall. Baptisia, Dictamus, or any other perennial with a long tap root may resent transplanting at any time. Yucca roots are so deep it can be hard to get them all. Bearded Iris prefer midsummer transplanting.
If the original soil was really good quality and nice and deep this job can be a breeze. If not then this is the time to amend the soil so next time it is easier. The job will be easier after a thorough rain or watering. Try to only attack what you can do in one day. A tarp beside the garden is useful to protect the lawn or path and make clean up easier. Start by assessing what is there, this is much easier in fall when perennials are full sized. Larger and specimen plants should be left in place unless the plan includes raising the soil more than 4”. Tying up the branches on shrubs and small trees can help to keep them from being damaged. Root systems for larger shrubs can almost equal the shrub size so they can be difficult to move if they have been planted for over 5 years. Many large shrubs respond well to drastic pruning, but wait till spring. See Shrub Pruning Guide.
If there are bulbs in the garden they can be re-planted (even in spring) after the soil is cleaned or simply left where you had them. Daffodil and Lily bulbs can be very deep if they have been there for years and often are difficult to find. Adding extra soil will not harm them.
You need to dig out plants that need dividing, weeding or moving. These can be left on the tarp for the day but shading them is a good plan. You are mostly preserving root for perennials, cutting off ½ or even 2/3 of the foliage will usually not harm perennials in fall transplanting. Shrubs however should not be cut back in fall as it may promote new unwanted growth. Dig through the soil and remove all the weeds and make sure to get the weed roots as well. Grass roots are easy to spot, and the more time you spend at this stage sifting out roots the cleaner the garden will stay. Sometimes it even pays to literally sift the soil you have (although with clay soil this might be difficult). Once you are convinced the weeds are removed, add any soil amendments and mix with existing soil. Use some of the new soil around existing plants you have left in place, being careful not to add more than 2-3 inches (5-7 cm).
Once this is done, you can add back the plants you have on the tarp, or add new plants to change things up. Add in some bulbs for better early spring colour. Water well after replanting, and expect perennials to look sad for the rest of the season, but they should be fine the next spring. A thick layer of mulch is a good idea in any garden to keep weeds suppressed and add organic matter.