get_post_custom()
is a very outdated method of getting post meta. To get the post meta properly you should use get_post_meta()
:
$my_custom_field = get_post_meta( get_the_ID(), '_gardener_address', true );
foreach ( $my_custom_field as $key => $value ) {
echo "<li>" . $key . " => " . $value . "</li>";
}