One of my current favourite foliage plants is lime-green Euphorbia oblongata which I grow from seed every year although it is actually a short-lived perennial.
It is easy to grow and has a long flowering time from about May through to late Autumn. Keep cutting off old stems when they go brown to encourage new growth and it will reward you by going on for months.
It is fantastic for splash of acidity amongst other rich plant colours in the border but really comes into its own when used in vases with other flowers. Be careful when you pick it as the sap often irritates skin, so wear gloves for picking and stripping the leaves. Make sure you sear the stems first and you will get close to 7 days of vase life from the stems.
Here seen with alliums early in the summer and then again with ‘Gertrude Jekyll’ roses later on in the year and eventually with Dahlias like “Chat Noir’ well into Autumn.