Heat and Growth Metrics: Reading the Season Through Temperature

If you track four numbers through the year, you can predict when your fruit trees will break dormancy, when the soil is warm enough to plant, whether your season is running ahead or behind, and how much solar energy your landscape is actually receiving. Those four numbers are growing degree days, chill hours, soil temperature, and sunshine hours. Each one tells you something different about where the season stands, and together they give you a picture of plant development that no calendar can match.

The weather dashboard tracks all four across six stations spanning the Puget Sound lowlands. This guide explains what each metric means, why it matters for growing things in a maritime climate, and how the differences between stations translate to real decisions in your yard.

Growing Degree Days: The Biological Clock

GDD is the accumulated heat that drives every phenological event in your landscape: bud break, bloom, pest emergence, fruit ripening. HortGuide has a full guide to growing degree days covering the calculation, our base-32 calibration, and the indicator species sequence. If you have not read it, start there.

On the dashboard, the GDD chart shows cumulative accumulation for all six stations since January 1. The station running highest is not necessarily “warmest” in the way you might expect. Seattle leads most years because of the urban heat island effect: pavement, buildings, and reduced canopy cover hold warmth through the night, keeping daily low temperatures higher than surrounding areas. That translates directly into faster GDD accumulation. Sequim, despite its reputation as the dry, sunny side of the Sound, often runs lowest because its clear skies allow more radiative cooling at night. Cold clear nights are a signature of the rain shadow.

The spread between the highest and lowest station tells you how much phenological variation exists across the region on any given date. In early March 2026, the gap between Seattle and Tacoma was nearly 90 GDD₃₂, roughly a week and a half of plant development. That is the difference between “forsythia is done” and “forsythia is just opening.” If you are timing a dormant oil application and your closest station is Bellingham, the Kent schedule may be a week too early for you.

Chill Hours: The Dormancy Requirement

Deciduous fruit trees need cold before they need warmth. Chill hours measure the time spent below 45°F during dormancy, typically November through February. Until a tree accumulates enough chill, it cannot break dormancy properly regardless of how warm spring gets. Insufficient chill produces delayed, uneven bloom, poor fruit set, and weak vegetative growth.

The threshold varies by species and cultivar. A low-chill apple like ‘Anna’ needs around 200 hours. A standard ‘Honeycrisp’ needs 800 to 1,000. Most sweet cherry cultivars need 700 to 900. Blueberries range from 150 (‘Misty’) to over 1,000 (‘Bluecrop’). The variety threshold lines on the chart below are not decorative. They mark the point where that group of cultivars has banked enough cold to wake up on schedule.

The maritime climate here delivers plenty of hours below 45°F in a typical winter. But “typical” is doing a lot of work in that sentence. The 2014-2015 winter was the warmest on record for the Puget Sound region at that point. Growers who had never worried about chill hours suddenly saw erratic bloom and poor fruit set on cultivars that had performed reliably for years.

The station comparison reveals a pattern worth understanding. Sequim and Kent consistently log the most chill hours because clear, dry nights (Sequim) and valley cold air pooling (Kent, in the Green River valley) both push overnight temperatures below the threshold. Seattle logs the fewest, again because the urban heat island keeps nighttime lows warmer. If you are choosing fruit tree cultivars and you garden in a warmer urban microsite, the chill hour data gives you a reason to lean toward lower-chill varieties rather than learning the lesson after three seasons of poor production.

The practical question chill hours answer: did my trees get what they needed? Check the dashboard in late February. If your nearest station has crossed the threshold for your cultivars, dormancy is satisfied and the tree will respond normally to the first sustained warmth. If not, expect a messy, protracted bloom.

Soil Temperature: The Underground Season

Air temperature gets the attention. Soil temperature drives the action. Root growth, nutrient uptake, microbial activity, seed germination, and the timing of soilborne pest emergence all respond to soil temperature rather than air temperature, and the two can diverge significantly.

The dashboard tracks soil temperature at 2-inch depth across all six stations. That depth is where feeder roots concentrate in turf, annual beds, and newly planted perennials. It is also the depth that responds most quickly to surface conditions, which makes it useful for planting decisions.

Three thresholds matter for practical horticulture in this region.

40°F is the temperature below which most cool-season root growth stops and soil microbial activity drops to minimal levels. When the soil line sits below 40°F, the ground is effectively dormant. Fertilizer applied to dormant soil does not get taken up; it sits there until conditions change, and in our wet winters, a meaningful fraction washes away before the soil warms enough to use it. This is why fall fertilization timing matters: you want the nutrients in the root zone while the soil is still warm enough for uptake, not after it has gone cold.

50°F is the transition point. Cool-season turfgrass roots are actively growing. Soil organisms are processing organic matter. Warm-season annuals can go in the ground (barely). When the 2-inch soil temp crosses 50°F and stays there, the growing season has started underground regardless of what the air feels like on a cloudy morning.

60°F opens the warm-season window. Tomatoes, peppers, squash, and warm-season turf species want soil at or above 60°F for planting. In the Puget Sound lowlands, this typically does not happen until late May or June, which is why experienced gardeners here laugh at the national “plant tomatoes after last frost” advice. Last frost might be mid-April. Soil at 60°F might be six weeks later.

The station spread on soil temperature is less dramatic than air temperature, because soil buffers thermal swings. But the differences that exist are meaningful. Sequim's well-drained, gravelly soils warm faster in spring than Kent's heavy clay. Seattle's urban soils, insulated by surrounding pavement and structures, hold warmth longer into fall. If you are making planting timing decisions, the soil curve for your nearest station is more useful than the air temperature forecast.

Sunshine Hours: The Energy Budget

Sunshine hours measure how long direct sunlight reaches the surface each day. This is not the same as day length (photoperiod), which is an astronomical constant for a given latitude and date. Sunshine hours are what is left after clouds, fog, and marine layer subtract from the available daylight.

In a region where overcast skies define half the year, sunshine hours are the metric that explains why the same species performs differently in Sequim than in Olympia. Photosynthesis runs on light, not temperature. A plant can have adequate warmth and water but still underperform if cloud cover limits the energy available for carbon fixation.

The dashboard uses a 7-day rolling average to smooth out the noise of individual cloudy days. The pattern that emerges is consistent: Sequim, in the rain shadow of the Olympic Mountains, logs substantially more sunshine than any other station. The gap is not small. Over a full growing season, Sequim may receive 15 to 25 percent more sunshine hours than Olympia or Tacoma. That is the energy equivalent of a full extra month of photosynthesis.

For the home gardener, this matters most for plants at the edge of their performance range. Lavender, rosemary, and other Mediterranean species that thrive in Sequim but struggle in south King County are not failing because of cold. They are failing because of light. Fig trees that produce reliable crops on the east side of the Sound may set fruit but never ripen it on the west side, because the sugar accumulation that drives ripening depends on intercepted light as much as on temperature.

The sunshine chart also reveals why early spring can feel slower than the GDD numbers suggest. March might deliver enough warmth to push GDD accumulation forward, but if most of those warm days are overcast, the photosynthetic engine is running at partial capacity. The plants are developing on the heat clock but not yet building the carbohydrate reserves that fuel vigorous growth. When the clouds break in May and June, the combination of accumulated heat and available light is what produces the explosive growth surge that Pacific Northwest gardeners know well.

How the Metrics Work Together

No single number tells the whole story. The value of tracking all four is in the relationships between them.

A warm winter with low chill hours and high GDD accumulation produces early bud break on species that met their chill requirement, but delayed or erratic bloom on high-chill cultivars. The soil temperature curve tells you whether the root system is ready to support that early top growth, or whether the canopy is going to push flush while the roots are still cold.

A cool, sunny spring (common when high pressure settles over the region in April) delivers moderate GDD accumulation, high sunshine hours, and rapid soil warming. Plants that break bud in those conditions tend to produce compact, well-hardened growth, because the light-to-heat ratio favors carbohydrate storage over elongation.

A warm, cloudy spring (marine air incursions in May) pushes GDD fast, keeps sunshine low, and produces the leggy, soft growth that is most susceptible to late-season pest and disease pressure. If you are seeing that pattern on the dashboard, it is a signal to hold off on nitrogen fertilization: the last thing that growth needs is more push.

The six-station comparison turns these patterns into a regional map. You can see which stations are running ahead on heat, which are banking more chill, which are getting the light. Over time, as the dashboard accumulates multiple years of data, those station-level patterns will become the most granular growing-condition reference available for the Puget Sound lowlands.

Getting Started

Pick the station closest to your location and check the dashboard once a week. You do not need to memorize thresholds or run calculations. Just watch the curves.

When the GDD line for your station crosses 600, your early-spring management windows are open. When chill hours plateau (they stop accumulating once nighttime lows stay above 45°F consistently), your fruit trees have banked what they are going to get. When the soil temp line crosses 50°F, the underground season has started. When sunshine hours start climbing in May, the growing engine has fuel.

If you want more precision, the plant profile pages show species-specific GDD thresholds tied to the season tracker. The dashboard gives you the regional overview. The profiles give you the plant-by-plant detail.

The season does not wait for the calendar to catch up. These four metrics tell you where it actually stands.

← Back to the weather dashboard

---

This guide supports the weather dashboard at /weather/ and links to the full GDD guide at /guides/growing-degree-days. All metrics are tracked daily from six Puget Sound stations using Open-Meteo historical and forecast data. Chill hour variety thresholds are approximate ranges compiled from university extension sources and vary by cultivar within each group. Soil temperature thresholds reflect general horticultural guidance applicable to Zone 8b maritime conditions.

Sources: HortGuide 6-station weather network (Open-Meteo API); WSU Extension fruit tree chill hour guidance; Oregon State University chill hour documentation for Pacific Northwest cultivars; UC Davis fruit and nut research chill requirement tables; HortGuide field observations, Kent, WA, 2025-2026 season.