How do you plant shrubs in clay soil on a hillside?
What is the best way to plant lilac, weigela, and spirea shrubs on a hillside? I have heavily clay soil on a slope that has a 10-20% gradient. I've heard some people say to create a berm on hills to help collect the water. I've seen other people say never create a berm in clay soil and that in fact I should do just the opposite and raise the root ball above the surface level. I've seen some websites say to amend the soil to help water retention and I've seen others say to leave the soil as is because otherwise the roots will get used to the good soil and not grow into the clay soil.
The area I'm planting in is not irrigated, so I thought I could cheat a bit and give the plants some needed extra water by creating a berm (something I saw a landscaper do) but now I'm unclear. I also have landscaping fabric below the mulch and I was worried that rainfall would just bead down the fabric instead of staying next to the plants. I know I'm going to need to irrigate somehow throughout the first growing season, but will I need to set up a drip irrigation for these plants longterm? I have established lilacs in the area that do fine as is, but the nearby viburnum had some droopy leaves this summer.
Also, at this point in the season, will it help to fertilize the new shrubs or will that just encourage unwanted new growth before the frost?
Washtenaw County Michigan
It is good you are thinking ahead so that you have the best chance at success. Anything you plant should be given water for as many seasons as it takes the plant to return to 'normal' growth; that is, lilacs average 12+ inches per year, returning to that amount would indicate the plant is well established. Also, all plants need to be watered during droughts, which we seem to be having repeatedly over the last several years. I would plan for a soaker hose system under the mulch (and any remaining landscape fabric) with 'quick connects' if you need to run a regular hose to them. Remove the landscape fabric if you can, it is no longer recommended because debris accumulates on top and weeds sprout in the debris. It also prevents organic matter from working its way into the soil below. You can build short terraces or individual reinforced areas to slow water down. Yes, you can place the rootball slightly above the soil level in clay. You can work some organic matter into the soil you replace in the hole, but keep it to less than 1/3 the total volume. Do not fertilize until next spring. Be sure everything goes into winter well watered, even if they have dropped their leaves. You should keep the soil moist until the ground freezes. This gives plants the best chance to survive the winter.
For your future internet research, here is a tip- to restrict the results to mostly university research-based sites enter your search with "site:.edu" on the end. For example "retaining walls steep slopes site:.edu"
University sites and non-profit organizations, like the American Horticultural Society, will give you research based info--- not all will agree with each other, but you will get a better idea of the latest science( do note the date on the articles you use, as the science does change). Choose sites with locations similar to Michigan growing conditions when looking for plant recommendations.
The points I gave you are explained in more detail in these links:
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