Spring Bloom in Massachusetts: Peak Timing for 2027

Arnold Arboretum of Harvard University puts the window at May 1 to May 28, with the peak around May 12, though that peak is the midpoint of their window rather than a date they publish. Their words: "provides several weeks of flowering in May." That is a climatological typical. The panel at the top of this page then reads USA-NPN's Spring Index for Boston, Worcester, Nantucket and tells you whether this particular spring is running ahead of or behind that.

No Spring Index anomaly is on file for Massachusetts for 2026. That number comes from the USA National Phenology Network's Extended Spring Indices, a set of 4 km gridded models regenerated every day from PRISM temperature data. USA-NPN describes the calculation plainly: to determine how the current spring compares to normal, they difference the day of year the leaf out or bloom was reached this year from the long-term average day of year it was met. We read the grid at Boston, Worcester, Nantucket and average.

No. Nothing USA-NPN publishes in this product suite is dated later than today. The Spring Index anomaly measures the spring that has already run so far this year. It is a leading indicator for the blooms still ahead of you, not a prediction of them. In the second half of the calendar year the live grid holds the spring that just finished, and this page stops shifting dates entirely rather than moving next spring by last spring's number.

USA-NPN publishes three, covering three different parts of spring. First Leaf is based on the leaf out of lilacs and honeysuckles, which are among the first plants to show their leaves in the spring. First Bloom is based on their flowering, and USA-NPN associates it with blooming of early-spring shrubs and leaf out of deciduous trees. The Late Bloom Index indicates when plants that bloom at the tail end of the season are flowering. Each species on this page is matched to whichever of the three normally fires closest to its own typical peak, and shifted by that one. At Boston the three normally land around day 96, day 129 and day 163 of the year.

Lilac: The Arnold Arboretum of Harvard University at Jamaica Plain, where the lilac collection on Bussey Hill runs to more than 400 plants representing 173 kinds. Daffodil: Nantucket island wide, along Main Street, Milestone Road and out to Sconset. Cranberry: The southeastern Massachusetts bogs around Plymouth, Carver, Wareham and Middleboro, and on Cape Cod. Those locations are the ones the sources named; where a source named a species but no place, or a place but no month, the card carries what the source gives.

It is an assumption. The Spring Index models three cloned ornamental cultivars, a lilac and two honeysuckles, chosen because they are among the first woody plants to leaf out and bloom and are common across much of the country. It has never heard of the species on this page. What the shift assumes is that a plant blooming at roughly the same point in the season responds to the same accumulated warmth in roughly the same way, which is why each species is matched to the nearest of the three indices rather than to one generic number. Elevation, aspect, and local soil moisture will all move the real answer, and a hard late freeze can take a bloom out entirely.

The windows on this page do not; they are climatological typicals compiled from published guidance and they stay put. What changes is the spring. That is the entire reason the Spring Index panel is here: the researched window tells you where to start, and the anomaly tells you which way this particular year has moved it. Any single season commonly runs a week to a fortnight either side even before you account for the anomaly.

Yes, and you should. The Spring Index layers are published by the USA National Phenology Network at usanpn.org under a Creative Commons Attribution 4.0 licence, and their map service is open, so the same numbers this page shows are queryable by anyone. The bloom windows come from the institution named under each species card, with the sentence we relied on quoted directly so you can find it on their page.