How to read this map
Two kinds of map
The badge above the map always names the one on screen.
Modeled is the strictest kind, and so far only the lilac has earned it. The layer is the Extended Spring Index for the indicator clone Syringa x chinensis 'Red Rothomagensis', one of the plants behind the national SI-x bloom layer, which averages three models rather than resting on a single plant, worked out for every cell from thirty years of daily station records (1991 to 2020). Before it went on the map it was checked against the USA National Phenology Network's own national index: the two disagree by 1.6 days on average across 791 stations. That figure is agreement with their index, not accuracy against a living lilac. The independent check of the Extended Spring Index published in Scientific Data in 2024 put its bloom layer 10.7 days from 15,833 dated US records, running about nine days early, and this layer is built on the same index and carries the same distance. That is why these maps print a date, and why In bloom on them is a real count, the share of those thirty years in which a place was in flower on the chosen day. If you grow the common lilac, Syringa vulgaris, expect it a few days either side of the clone.
Typical window is the kind every other flower carries. It starts from dated bloom records: Nature's Notebook observations reduced to one row per site with the mean first-flower day and its spread, plus botanical garden bloom reports, trial publications and herbarium records where they exist. Those dates are fitted, as a straight line, to a heat sum, the growing degree days above 50 F counted from the last spring frost. The heat sum is already known for every cell, so the fitted line carries the dates across the country. It is not an interpolation of the observations across space, and it is not a forecast for any one year. The width of the window comes from the scatter of the sites and of the years, and a window is allowed on the map only if it covers every dated in-bloom record held for that flower, including records left out of the fit. Cherry, dogwood and magnolia failed the stricter tests a modeled layer has to pass and stay as windows. The colour follows the middle of the window; the tooltip prints the whole of it for any point you touch. In bloom works here too, without percentages; the tone simply rises towards the middle.
The colour is the calendar
The colour of a day is the same on every flower. The scale is the calendar itself, the whole year of it, so April on the lilac is the April on the magnolia. The legend shows the full year and lifts the veil over the months the flower on screen actually uses.
What the hatching says
Hatched ground marks the edges of the plant's range, and its language is the same on every flower. Clay-coloured lines are the warm side: winters too mild, or the wrong summers. Slate-coloured lines are the cold side: winters or frost the plant does not get past. Lines over colour mean the flower grows here at its limit and a date still stands; lines over bare paper mean the map refuses to print a date at all.
Ink dots are about the data, not the plant: high ground keeps its colour and its date under dots because the reading up there has less to lean on. Flat grey means no records and no claim either way. The legend lists all six signs in the same order on every flower, and dims the ones the current map does not use.
Built from NOAA GHCN-Daily station records (1991 to 2020) and dated bloom observations. Data were provided by the USA National Phenology Network and the many participants who contribute to its Nature's Notebook program. Dates are averages: cultivars of the same flower can open days or weeks apart.
Who made this map
Built and kept up by the Your Flowers Guide editorial team, from published temperature records and dated bloom observations.
Found something wrong for your area? Write to [email protected] with the place and the flower, and it will be checked.
The map, its images and the text on this page may be quoted and shared with credit and a link to this page. The underlying grid is not offered for redistribution.