Answer: It is possible that they are suffering from underwatering, especially if the potting mix dries out much faster than they surrounding soil. However, your description of deterioration after rain and watering could certainly indicate overwatering, possibly due to a soil problem; these plants require a well drained location and leaf drop can be a definite sign of overwatering. It can also be a sign of planting too deeply, meaning deeper than they grew originally in the container, or planting them in a poorly drained low spot or location that receives heavy runoff such as from a rainspout.
It can also happen that the planting hole is working as a sump, holding excess water around the roots. This can happen if your soil is a heavy, clay based soil that drains much slower than the potting mix. A raised bed can help with this problem, as can adding ample amounts of organic matter plus a small amount of course sand or grit to amend the soil over the planting bed rather than in just the planting hole.
It is also possible that for some reason they have failed to root out into the neighboring soil. A failure to root out of the original potting mix can be caused by poor planting technique such as allowing encircling roots to remain rather than untangling them or cutting some and directing them outward, planting too deeply, or an unamended clay soil working to contain the roots as though in a pot underground.
You might want to dig one up and see what is going on underground, make any necessary corrections and replant either there or in a new better location. Water to keep the soil slightly moist but not sopping wet.
To judge if and when to water, use your finger and dig down an inch or two into the soil. Check both the potting mix and surrounding soil, sometimes they dry out at different speeds. Your goal is damp but not soaking wet or saturated.
Last but not least, defoliating can happen as a response to chemical sprays or excess fertilizer, or even a water soluble fertilizer applied at high temperatures. Always be sure to read and follow label instructions.
I hope this helps you trouble shoot.
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