The following is a general guideline of the flowering season for peonies in the UK. Weather conditions and specific locations will affect timing. Cooler areas could be a week or two later and warmer areas could be a week or two earlier. 

Very Early Season - flowering late April

Early Season - flowering in early May

Mid Season - flowering mid-late May

Late Season - flowering early June

Itoh peonies, also known as intersectional peonies, are a unique type of peony hybrid resulting from the cross between a tree peony and a herbaceous peony. These plants inherit desirable traits from both parent types, such as the large, fragrant flowers of tree peonies and the compact, herbaceous growth habit. They are known for their prolific blooming, sturdy stems that don't require staking. They are generally more resistant to fungal diseases compared to herbaceous peonies. 

Lactiflora Peonies are herbaceous peonies that are one of the most easily grown hardy perennials available today.  Long lived and durable, a peony can easily live as long as 100+ years. They are often referred to as the Chinese Peony, which continues to be commonly used in the trade today.   Lactiflora cultivars often have sweet fragrance and generally make outstanding cut flowers.

Hybrid Peonies have recently been created by peony hybridizers by cross breeding various species to create hybrid peonies.   The hybrids exhibit a wide range of both flower and plant characteristics, which lend themselves to more diverse use in the garden and landscape.  Both P. lactiflora and the hybrids are grown in the same way, have similar plant habits and cultural requirements.