Thin and stake out Herbaceous Perennials.
Thin out rose growth and watch out for Greenfly especially on the young growth of roses and fruit trees. You could also spray roses early to prevent black spot and apple trees against scab.
Kill slugs, these will eat your tender seedlings, but if you are a dog owner and do not treat your pet with Advocate they can spread a lung illness, that can be fatal to dogs, speak to you vet about this.
Feed plants that are in full growth.
Plant Aquatics May is the best time to plant water lilies.