Rosemary,
A lot depends on whether the subsoil can actually drain. Very few plants will do well if they have wet feet due to inadequate drainage. Compost can only do so much but does very little to address your underlying clay soil at deeper levels. Typically clays are good to build on due to favorable compaction, and quite often they form over time an underlying hardpan. Hard pans are just that and are very solid and hard to dig thru and can prevent drainage of the topsoil. Chances are the builder compacted the underlying soil before building the garage you removed.
You should first investigate your drainage by conducting a simple perc test. To do this dig a 2’deep hole, fill it with water and observe how quickly it drains. If you have excellent drainage the water should soak in within 2 hours. If it takes a day or more, then there is a need to determine why, before planting your trees. Sometimes a hardpan in clay soils can be penetrated allowing drainage to occur if it is shallow enough. In any event sometimes it’s essential to install a drainage trench with perf pipe to allow an area to freely drain. If this is necessary, it’s better to do it before you have trees involved. Heavy clay soils can be softened up if you dig your compost to deeper levels, but that may require tillers that can mix the clay with soil amendments. Clays soils can also be a source of improving soil fertility when compost is incorporated greatly improving the CEC (cation exchange capacity) of your soil.
Bottom line: do a perc test and make sure you have good drainage before planting.
Dennis
Kent, wa
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