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Field Brief

May 2026 · HortGuide Station Network · Puget Sound Lowlands

Weather Report

April Recap

April was two months in one. The first half hit 76.7°F at Kent on April 6, pushing GDD accumulation to nearly 21 GDD₃₂ per day. Then a convergence zone collided with cold air from the Gulf of Alaska on April 14-16, dropping the snow level to 500-1,000 feet. Issaquah picked up 0.62 inches of snow, Seattle saw hail accumulating in several neighborhoods, and a waterspout formed over Puget Sound west of Magnolia. It was the first measurable April snow at Sea-Tac since 1972. Olympia recorded the network's only below-freezing temperatures: 30.9°F on April 8 and 32.0°F on April 16. If you garden in the south Sound, those Deschutes valley frost pockets are real and worth mapping.

The second half recovered fast. Kent hit 79.7°F on April 19, and the post-storm stretch was predominantly dry, opening spray windows across the network. Kent and Seattle both touched 60°F at 6 cm soil depth on April 20 before dropping back to the mid-50s, the first signal that the warm-season planting window is approaching. Sequim, the last holdout below 50°F, finally crossed that root activity threshold on April 19. All seven stations are now above 50°F. The month closed with another warming push: Kent reached 69.3°F and Olympia 69.7°F on April 29, and no station recorded measurable precipitation over the final three days. After the mid-month storm, April delivered 13 straight dry days at Kent to close out the month.

1,670

GDD₃₂ (Kent)

607

April GDD₃₂ (Kent)

56.9°F

Soil Temp 6cm (Kent)

21/29

Spray Days (Kent)

1.51"

April Precip (Kent)

113

Field Observations

Station Data

By the Numbers

Station data through April 29, 2026. Sorted by GDD₃₂ so you can see where your area sits in the season.

Station	GDD₃₂	Apr GDD	Soil 6cm	Spray	Precip
Issaquah	1,719	587	55.4°F	14/29	2.20"
Seattle	1,676	551	57.2°F	18/29	1.63"
Kent ★	1,670	607	56.9°F	21/29	1.51"
Olympia	1,626	564	52.0°F	16/29	1.44"
Bellingham	1,615	562	54.3°F	21/29	0.87"
Tacoma	1,595	566	55.6°F	16/29	1.78"
Sequim	1,558	535	52.0°F	20/29	0.47"

Issaquah leads on cumulative GDD but has the fewest spray days and highest precipitation in the network. GDD tracks biology; spray days track operational conditions, and a station can lead on one while trailing on the other. Kent accumulated the most heat this month at 607 GDD₃₂. Bellingham closed the gap with Kent from 107 GDD in March to 55 now. If you manage properties north of Everett, use the GDD column rather than the calendar to time your work.

Phenology

Field Notes: April Phenology

I logged 113 observations across Issaquah, Kent, and Seattle in April. Here are the highlights.

Bigleaf maple Acer macrophyllum

Broke bud April 6 in Issaquah at 1,254 GDD₃₂ and reached flower bud opening three days later at 1,308 GDD₃₂. Just 54 GDD₃₂ between stages. When bigleaf maple leafs out, the understory light environment changes completely, and species that depended on early-spring sun lose their window.

Native species wave

April 9 brought four native species to the same developmental stage in Issaquah, all at approximately 1,308 GDD₃₂: vine maple at leaf emergence, western sword fern pushing fiddleheads, red alder with leaves emerging and catkins shedding, and evergreen huckleberry in bloom.

Red maple Acer rubrum

Leaf emergence April 13 in Issaquah at 1,393 GDD₃₂. The National Phenology Network's predicted threshold is 1,364 GDD₃₂, putting our observation within 2%. This is the calibration work the observation network is designed to produce.

Oregon grape Mahonia aquifolium

Progressed from early bloom (March 21, 915 GDD₃₂) to full bloom (April 9, 1,242 GDD₃₂) in Kent: 327 GDD₃₂ and 19 days between stages. Between-stage intervals tell you the pace of development, so you can estimate where a species is now without checking a calendar.

Japanese maple Acer palmatum 'Crimson Queen'

Leaf emergence April 9 in Issaquah at 1,308 GDD₃₂, 20 days and 350 GDD₃₂ after bud break on March 20. Tracking a single cultivar across stages makes next year's timing more predictable. Japanese maple selection →

Apple bloom confirmed

Three cultivars hit pink bud April 18 in Kent at approximately 1,385 GDD₃₂: 'Red Jonagold,' 'Chehalis,' and 'Liberty.' By April 27 (~1,624 GDD₃₂), Chehalis had reached full bloom and Red Jonagold was beginning to open. Staggered cultivar timing extends the bloom window, which matters for pollination and for the period flowers are vulnerable to bacterial disease.

Lookahead

May: What to Watch For

Warm-season planting: know your soil, not your calendar

Kent and Seattle briefly touched 60°F at 6 cm on April 20 before dropping back to the mid-50s. At current warming rates, sustained 60°F is likely mid-May for the leading stations, late May for Olympia and Sequim. But 60°F is only the floor, and not all warm-season crops share the same threshold. Tomatoes survive at 60°F but establish better at 65°F. Squash and cucumbers want 65-70°F. Peppers and basil need 70°F or warmer. A soil thermometer pushed 2 inches deep for direct-sown seed (4-6 inches for transplants) is more reliable than any calendar or forecast. Cold roots do not just grow slowly; they cannot absorb nutrients or water efficiently, so the plant sits stressed and vulnerable instead of establishing. OSU Extension recommends adding 10-14 days to seed packet maturity dates for our cooler growing conditions. The impulse to plant everything on the first warm weekend is strong. The soil thermometer is the counter-argument. Spring vegetable planting →

Fruit tree thinning: the window that determines your harvest

With Chehalis apple in full bloom and Red Jonagold opening, petal fall is days to weeks away. When it arrives, the thinning clock starts. For apples, thin within 40 days of petal fall to one fruit per cluster, keeping the king bloom (the center fruit that opened first). For pears, the window extends to 60 days, one fruit per spur. Cell division in developing fruitlets lasts four to five weeks after petal fall, and that division period is when thinning has maximum effect on final fruit size. Once cell division ends, the relative sizes on the tree are fixed. This is not cosmetic pruning. It determines whether your tree produces fruit worth eating, and it reduces biennial bearing. If you miss the window, you wait a full year for the next chance.

One note on spring shoot blight: if you see blackened shoot tips on apple or pear later this month, the most likely culprit in this region is Pseudomonas blossom blast, not fire blight. Pseudomonas thrives when frost during bloom creates entry points for bacteria, and Olympia recorded 32.0°F on April 16 during the bloom period. Fire blight requires open flowers plus temperatures above 65°F plus a wetting event of two to three hours, conditions that rarely align here. Diagnosing spring shoot blight → Fire blight bloom management →

The landscape is accelerating

Everything is growing, and May is when the maintenance calendar catches up. Mowing frequency ramps to four to six times per month at 3 to 3.5 inches. Broadleaf weed control (dandelion, clover, plantain) is most effective from May through mid-June, when weeds are actively growing and absorbing herbicide at their highest rate. That spring flush of weeds is actually working in your favor for timing.

Spring-blooming shrubs are entering their after-bloom pruning window. Forsythia is finished across the network. Pieris was fading by April 9. Lilac should be pruned immediately after bloom. These species set next year's flower buds on old wood, which means dormant-season pruning removes future flowers. The window is narrow: prune within a few weeks of petal fall, then leave the plant alone.

April was drier than usual through the first 13 days, and the post-storm stretch from April 16 onward stayed mostly dry. Soil moisture reserves may be lower than expected heading into May. If established plants show stress before May rain arrives, supplemental water is reasonable even this early in the season.

Scouting your plants: two diseases, two different conditions

Two diseases are both finding favorable conditions right now, through different mechanisms. Botrytis (gray mold) needs four to six hours of wetness at 60-77°F to infect, and in the Puget Sound lowlands, nearly every rain event during bloom meets those conditions. Strawberries, blueberries, peonies, and roses are all susceptible while in flower. The management is cultural: deadhead spent flowers promptly, switch to drip irrigation before bloom, avoid overhead watering, and improve air circulation in dense plantings.

Powdery mildew works the opposite way. It does not need free water at all; rain actually suppresses spore germination. It thrives in the warm, dry stretches between rain events at 60-80°F with moderate humidity. Scout undersides of new leaves on roses, dogwoods, and susceptible apple cultivars. What you find or miss on new growth in May drives your mildew pressure for the rest of the summer. The maritime climate gives us one advantage most gardeners elsewhere do not have: fall rains suppress powdery mildew by late August, compressing our management window to roughly six to eight weeks. Botrytis blight → Powdery mildew →

What is blooming and why it matters

This is the most dynamic four weeks of the growing season. Apple and pear bloom is active. Rhododendrons and azaleas are in full color. Lilacs are approaching peak. Bigleaf maple canopy is closing. Vine maple is leafing out. Oregon grape has finished bloom and is setting fruit. Blueberry bloom is arriving. The landscape is hitting a pace where something new opens every few days, and paying attention to what is blooming is a skill worth developing.

These are your indicator plants, the phenological markers that tell you where you are in the season more reliably than any calendar date. When lilac blooms, crabgrass germination is approaching. When apple petals fall, your thinning clock starts. When rhododendron finishes, deadhead before energy goes to seed production. Noticing what is happening around you is the skill that makes everything else in this newsletter more useful. Growing degree days →

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