Frost will only affect what it can reach. So if you only have a light frost, perhaps sprinkling the top of the soil with silver but not freezing it solid at the depth of your bulbs/corms/tubers and any little shoots that might have started to grow from them, they should be okay. If you have a sudden heavy frost forecast, you can cover with fleece or a little more earth for safety. (I wish I had last frosts in April! Freesia are a heated-greenhouse thing here.)