not normally necessary for this plant.

Not necessary, but once they've been flowering and start fading (or before don't matter) you can give them a light "haircut" all the way around. They will reward you with with an ever expanding mound of blooms :D
They are great re-seeders too! I've only had to start them once and can be assured to find cute little babies (often too many, but easily thinned) each spring.

Sweet Alyssum and Catmint August 28th:
{{gwi:211836}}
Some self-sown babies...{{gwi:211837}}

I have a question. When you start alyssum do you separate all the little seedling into individual little cell pots or do you plant in clumps? I bought seeds to start and in the past I have used clumps but I'm thinking I'm doing it wrong.

This is a bit of a non sequitur, but I just thought I'd mention how much I love this flower because, though delicate and in scent and looks, it lasts for a long while even through frosts. :) This year I'm planting more!