For the past several months we’ve been working through a research project for the University of Montana Master Beekeeping Program, looking at what actually drives nectar production in the four plants that matter most to a honey flow around here: black locust, tulip poplar, basswood, and white clover. Some of this already made the rounds on the beekeeping Facebook group, but we wanted to write it up properly here, with a link to the full paper for anyone who wants the sources.

The short version: beekeepers talk about the weather affecting a nectar flow, but rarely get specific about which conditions matter for which plant. We went through the published research on these four species — 82 sources in all, going back to a 1933 tulip poplar study — to see what’s actually been measured, and to test something we hear a lot around here: that a hard rain “washes out” the nectar and a flower needs a day or two to recharge.

Rain Dilutes, It Doesn’t Wash Away

Where this has actually been tested, the answer isn’t wash-out. It’s dilution. For basswood, a Polish field study found that light rain thins the nectar rather than rinsing it out of the flower, an effect tied to basswood’s flowers hanging downward in the cluster. White clover’s numbers point the same way: growth-chamber sugar readings run 59 to 73 percent, against 20 to 57 percent measured out in the field, and researchers attribute most of that gap to rain and dew that never reach an indoor plant.

Black locust is messier. A 2026 Hungarian study measured rainfall in the week before sampling at three different sites and got three different answers — more rain meant more nectar weight at one site, less at another, and no relationship at a third. Nobody has run a direct wash-out test on tulip poplar at all.

Put together, this argues for a different read on a rained-on bloom than the one that gets passed around at bee meetings. A flower that’s been rained on is probably still worth the bees’ time, just weaker and wetter than it would be dry, not empty.

Heat, Not the Calendar, Drives Black Locust

Locust is the plant most of us obsess over every spring, since it’s the earliest big flow and the most unpredictable one. The research shows its bloom timing tracks heat accumulation, not a fixed date, and rainfall has no significant effect on whether or when it blooms. Running a heat-accumulation model against ten years of Missouri temperatures predicts full bloom around May 6, which is close to what we see locally. Checked year by year against our own bloom records from the past decade, though, the model still missed by an average of five days and by as much as two weeks in one year. Temperature is the driver. It’s just not one you can pin to the calendar in advance.

The Real Problem Is How Short the Window Is

The bigger issue for a beekeeper isn’t really the weather mechanism, it’s how little room these plants leave for bad days. An individual black locust flower secretes nectar for about three days, and the whole bloom only runs seven to ten days. Tulip poplar is worse at the flower level, secreting for only about a day and a half per bloom, even though the tree itself flowers over roughly three weeks. We’ve watched our own bees stay on locust and ignore tulip poplar entirely when the two bloom windows overlap, the same thing a 1933 USDA study documented near Washington, D.C.

White clover’s individual flowers are just as short-lived, but the plant blooms from March through November here, with our working flow usually falling in the first weeks of June into early July and multiple peaks rather than one flush. A bad week during that stretch has a real chance of getting made up somewhere else in the season. Locust and tulip poplar don’t get that chance. A couple of bad days can cost most of a season’s crop from either one.

What This Means for How We Manage

The practical takeaway for us is to stop treating any of this as a fixed schedule. Locust blooms on heat, not a date on the wall, and once a short window like locust’s gets cut short by weather, there’s little chance of recovering that crop in the same season. The better move is to stop managing for that crop and shift to whatever the next goal is, whether that’s splits, queen rearing, or getting ready for the next flow, rather than waiting on a recovery that isn’t coming.

It’s also worth knowing that bloom dates are moving. A Franklin County study of native wildflowers found the community’s spring flowering peak has shifted about 17 days earlier over the past century. Black locust itself hasn’t shown a significant shift yet, but the plants it flowers alongside are drifting earlier, which is one more reason to manage to what’s actually happening at the hive rather than to a bloom calendar from ten years ago.

Reading the Full Paper

This was a literature review, not a new field study, so the real value is in seeing what’s actually been measured versus what beekeepers just assume. We used some AI tools to help track down sources and translate two Polish field studies that only had English summaries, but every citation and number was checked by hand against the original source before it went into the paper.

Two questions came out of it that nobody’s answered yet: whether the way these flowers are built, locust and clover keep their nectar enclosed while tulip poplar sits wide open, actually changes how much rain affects them, and what black locust looks like in its native range, since almost all the published research on it comes from Europe and Korea, where it was introduced.

If you want the full paper with all the sources, you can read it here.