Short answer: Hendricksons are medium mayflies in hook sizes 12 to 14 that open the dry fly season across the East and Midwest. They hatch in the early afternoon when water reaches about 50°F, usually mid-April to early May, and they're the first hatch of the year that brings trout up with any consistency.

For eastern anglers this is the one that ends the winter. There's a specific afternoon every April when the water finally hits temperature, the duns start coming, and the river wakes up. Being there for it is most of the skill.

What a Hendrickson actually is

The Hendrickson is Ephemerella subvaria, a close relative of the PMD and the sulphur — same genus, same crawler nymph, same general behaviour, just earlier and a shade larger.

One quirk trips people up: the males and females look different enough to have earned separate names.

Female (the "Hendrickson") — size 12 to 14, pinkish-tan to reddish-brown body, grey wings.
Male (the "Red Quill") — slightly smaller, distinctly reddish-brown with a slimmer body.
Wings — upright, medium to dark grey, opaque on the dun.
Nymph — a stout crawler, dark brown, living in moderate riffles and runs.

If you carry both a Hendrickson and a Red Quill you're covering one insect twice, which is exactly what the old pattern names were built to do.

Fig. 1 — The Hendrickson cycle. In cold April water, stage 2 lasts a long time.

Key to the stages

Nymph

Dark brown crawler, sizes 12-14. Riffles and runs. Best fished the week before the hatch.

Emerger

Cold water means a long, slow struggle in the film. The stage to fish first.

Dun

Female pinkish-tan, male reddish (the Red Quill). Grey opaque wings.

Spinner

Mates over riffles in the late afternoon. Clear wings, rust body.

Spent spinner

Falls at dusk, often on cold evenings when few anglers stay out.

The 50-degree rule

Hendricksons are one of the most temperature-reliable hatches there is. When water hits roughly 50°F for a couple of consecutive afternoons, they come. Carry a stream thermometer in April and you'll stop guessing — it's the cheapest piece of gear that will change your season.

When it happens

Mid-April through early May across most of its range, running later further north and at higher elevation. Emergence is concentrated in the early afternoon — roughly 1pm to 4pm — because that's when a cold spring river is warmest.

Region	Mar	Apr	May	Jun
Pennsylvania — limestone and freestone	On	Peak	On
Catskills — Beaverkill, Delaware		Peak	Peak
New England		On	Peak	On
Michigan / Wisconsin		On	Peak	On
Southern Appalachians	Peak	Peak	On
Ontario / Quebec			Peak	On

Peak Active

This is an eastern and midwestern hatch. Western anglers get the same job done by Drunella species — the western Green Drake and the small blue-winged olives — so if you fish the Rockies this article is background rather than a calendar entry.

How to fish it

Go at lunchtime. Mornings are dead. The hatch needs the day's warmth, and a 1pm start puts you there for the beginning rather than the end.

Nymph the week before. As the hatch approaches, nymphs become restless and available. A dark crawler nymph through riffles in early April is excellent fishing and nobody's doing it.

Start with the emerger. Cold water slows everything. Duns take a long time to get free, and trout key on the struggling stage almost immediately.

Carry both colours. Female pinkish-tan, male reddish. If fish refuse one, try the other before you change size — they're feeding on both.

Stay for the spinner. The fall comes at dusk on a cold April evening, which is precisely when most people have gone for a beer. It's often the best half hour of the day.

Common mistakes

Fishing mornings. The most common Hendrickson error. Sleep in, eat lunch, then go.

Ignoring water temperature. Guessing at the calendar instead of measuring the water wastes days every spring.

Fishing only the dun. In cold water the emerger produces more, more often.

Leaving before dusk. The spinner fall is the reward for being cold and patient.

Flies to carry

We stock this insect specifically. The Hendrickson CDC Emerger covers the stage that matters most in cold April water, and the Hendrickson Hatch BugCycle carries nymph through spinner in one pack when it's in stock — restocks are seasonal, so check before you plan a trip around it. If it's out, the Trico Mayfly BugCycle teaches the same life-cycle lesson on a different mayfly.

Add the Bead Head Pheasant Tail Assortment for pre-hatch nymphing, the Soft Hackle Partridge and Pheasant Tail for the film, and the Adams Classic Dry Fly as a generalist dun. For the dusk fall, the Hi-Vis Rusty Biot Spinner is the one you can still see. See all fly pattern assortments.

April water is genuinely cold and you'll be in it for hours — see waders and wading jackets and how to stay warm when fishing cold. Use Loon Dust rather than gel, which thickens in the cold.

The bottom line

The Hendrickson is the hatch that reopens the season. Watch the thermometer rather than the calendar, go after lunch, start with an emerger, and stay until dark. Do that for a fortnight in April and you'll have the best early-season fishing available anywhere east of the Rockies.

More hatches: the March Brown, the Pale Morning Dun, the Trico, and the Blue Winged Olive.

Christian Bacasa, host of the Fly Fishing Insider Podcast