It's fine if you're a traditional bride at heart and you want your guests in their formal wear best. But if you're a summer bride getting married in a garden at noon, rethink your attire expectations.
Address wardrobe options via an invitation insert. You can choose your own wording, but you might say something clever like "Stiflingly hot jackets and ties forbidden!" or "Lightweight clothing advised." In my humble opinion, I think people will be so grateful for this reprieve that you'll benefit exponentially in terms of the quality of gifts. As an added bonus, fewer people will duck out early to shed their sweat-drenched attire.