z-index a pseudo-element box-shadow behind a parent element

I'm trying to place the :before,:after box-shadow behind the button. But the transition is starting in front of the a tag. For my works CMS, I'll need all the properties to be on the a tag.

<a href="#" class="btn">Join Today</a>


.btn{
  position: relative;
  z-index: 1;
  display: inline-block;
  text-align: center;
  background-color: #8ec656;
  border-radius: 30px;
  font-family: 'Roboto Condensed', sans-serif;
  font-size: 20px;
  color: #fff;
  border: 3px #d2d2d2 solid;
  padding: 15px 40px;
  line-height: 1;
  transition: box-shadow .4s;
}
.btn:hover{
  background: #6b9640;
  box-shadow: inset 0px -22px 13px 0px #84b652, inset 0px 2px 5px 0px #84b652;
}
.btn:before,
.btn:after{
  content: "";
  position: absolute;
  z-index: -1;
  bottom: 9px;
  left: 10px;
  width: 88%;
  top: 73%;
  transition: box-shadow .4s;
}
.btn:hover:before,
.btn:hover:after{
  box-shadow: 0 15px 10px #777;
}

https://jsfiddle.net/3aj5muyu/

Solution

.btn:before,
.btn:after{
  transition: .6s;
  content: "";
  box-shadow: 0 28px 17px -3px #777;
  width: 84%;
  height: 11px;
  top: 48%;
  left: 15px;
  position: absolute;
  opacity: 0;
}
.btn:hover:before,
.btn:hover:after{
  opacity: 1;
}

This is what I had to settle with. Thank you for your responses!