Monarch Migration in New Hampshire: 2026 Timing

The autumn southbound movement through New Hampshire typically builds from August 16, peaks around September 13, and thins out by October 25. That window is derived from 2,617 research grade iNaturalist records of adult monarchs in New Hampshire, divided by all observations in the state in the same week so it measures butterflies rather than how many people were outdoors. Monarch Watch, which models the migration from recovered tagged butterflies, puts the midpoint at latitude 43 at Sep 11. Our own curve and Monarch Watch land within 2 days of each other in New Hampshire, from two datasets that share no observations and no method, which is about the strongest thing either of us can say about a date like this.

Adult monarchs are typically being recorded again in New Hampshire from about June 14, and are established by June 28. Spring is not one arrival the way autumn is one departure. USFWS states that "As temperatures warm at the overwintering sites in the spring, monarchs begin to breed and lay eggs on milkweed throughout their spring journey." so the butterflies that reach New Hampshire in spring are usually the children or grandchildren of the ones that left it last autumn, not the same individuals.

Not in the legal sense, and this gets stated wrongly almost everywhere. US Fish and Wildlife Service proposed listing the monarch as threatened on December 12, 2024, with a section 4(d) rule and proposed critical habitat. As of Aug 24, 2026, no final rule has been published, so there is no federal listing in New Hampshire or anywhere else. The monarch is very widely described online as endangered. Under United States law it is not. The International Union for Conservation of Nature, which is a different body with no legal force in the US, assessed the migratory monarch separately, and its assessment has itself been revised. Those two things get merged constantly. This site states the federal position and names it as the federal position.

The milkweeds actually recorded in New Hampshire most often are common milkweed (Asclepias syriaca), swamp milkweed (Asclepias incarnata), butterfly milkweed (Asclepias tuberosa), poke milkweed (Asclepias exaltata). Xerces states: "The Xerces Society recommends planting milkweed species that are locally native and for which regionally-sourced seeds and plants are available." That list is what the observation record says grows here, which is a reasonable starting point for what is locally native, and the Xerces Milkweed Seed Finder will tell you who sells seed of it.

From about September 25 onward, which is roughly this state's median first freeze. Xerces states: "In most regions, fall planting is a good approach because the seeds' exposure to cold, moist conditions during winter and spring will help stimulate germination." So the fall sowing date is not a separate fact to look up, it is your frost date. New Hampshire's median first freeze is September 25 according to NOAA normals for representative stations. Xerces also states: "Following site preparation to remove weeds, seeds can simply be scattered onto the soil surface by hand."

Because a sightings map and a typical window are different objects. Journey North plots what volunteers reported this year, live, which is the right tool for "where is the front today" and which we link to on this page. This page publishes what usually happens in New Hampshire, from the observation record, so it can answer the question before this year's sightings exist. Two corrections move our dates away from the numbers you would get from a raw record count. Records are restricted to adults, because larvae and eggs peak weeks earlier: the unfiltered New Hampshire curve peaks in August and the adult curve peaks in September. And counts are divided by all observations in New Hampshire that week, which removes the effect of how many people were outdoors with a phone.

Mexico. New Hampshire is east of the Rockies, and USFWS states that in the fall monarchs "In the fall, they may fly more than 2,000 miles (3,000 km) to reach overwintering sites in Mexico." The destination is the oyamel fir forest of Michoacan, and the generation that makes that flight is the long lived one: "Most breeding adult butterflies live approximately two to five weeks, but overwintering adults that enter into reproductive diapause can live six to nine months."

No, and the numbers say so consistently. Regular adult monarch records in New Hampshire run to about October 25, while the state's median first freeze from NOAA normals is September 25, 30 days earlier. That pattern holds in every state on this site, which is why the practical advice is not to cut the garden back at the first frost. A first freeze is a light radiational frost, not the hard freeze that ends a season, and the autumn generation is still moving through on its way to central Mexico. Late nectar in New Hampshire is doing real work well past September 25.

Yes, and we use it as the check on our own numbers. Monarch Watch publishes midpoints of the autumn migration by latitude, derived from recovered tagged butterflies. The published row nearest New Hampshire's populated centre at 43.48 degrees north is latitude 43, with a midpoint of September 11. Our own curve and Monarch Watch land within 2 days of each other in New Hampshire, from two datasets that share no observations and no method, which is about the strongest thing either of us can say about a date like this.