FITZWILLIAM, N.H. (AP) — The flowers are blooming at New Hampshire’s Rhododendron State Park.

Bushes of Rhododendron maximum, also known as great laurel or rosebay, are opening their buds, with most expected to be blooming by Tuesday July 15 in a 16-acre grove in Fitzwilliam, in the southwest part of the state. They bloom just once a year.

The plants are at least several centuries old and are rare in New England. This is the largest known group of them in the region. It was designated a National Natural Landmark in 1982.

Rhododendron maximum are commonly found in the southern and central Appalachians along rocky wooded slopes.

In the New Hampshire park, their growing conditions, undisturbed through the years, are the key to their longevity: a swampy, evergreen forest with an acidic soil and enough shade to protect them.

“The warm temperature has sped up the process of all plant growth everywhere, including the rhododendron grove,” the state Parks and Recreation Department said in a recent report.

The report said it is normal to see some fully open flowers sooner than others. Some flowers will be much later than others, as well.

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