Water and Disease Metrics: Moisture, Evaporation, and What Makes Plants Sick

Every fungal disease that hits your landscape needs the same thing you do: water. The difference is that you need it in the root zone and the fungus needs it on the leaf surface. Understanding how water moves through your landscape, how much the atmosphere pulls back out, and when conditions favor the pathogens over the plants is the foundation of preventive plant health care. It is also the difference between irrigating on a schedule and irrigating based on what is actually happening.

The weather dashboard tracks five water-related metrics across six Puget Sound stations: precipitation, evapotranspiration, the water balance between them, dew point, and frost events. This guide explains what each one measures, why it matters for plant health in a maritime climate, and how to use the station comparisons to make better decisions about irrigation, disease management, and frost protection.

Precipitation: When It Falls Matters More Than How Much

The Puget Sound lowlands receive 35 to 50 inches of precipitation in a typical year, which sounds generous until you learn that roughly 75 percent of it falls between October and March. The growing season, when plants actually need the water, gets the remaining quarter.

The precipitation chart shows this pattern clearly. Daily bars stack up through winter, then thin out dramatically from June through September. The cumulative line, which tracks total precipitation since January 1, climbs steeply through spring and then flattens to nearly horizontal during summer. That flat section is the irrigation season, whether you realize it or not.

The station comparison reveals something less obvious. Sequim, sitting in the Olympic rain shadow, receives roughly half the annual precipitation of Olympia. That is a difference large enough to change which plants survive without supplemental water. A garden in Sequim operates closer to a Zone 8 Mediterranean climate than a Pacific Northwest maritime one. A garden in Olympia or Tacoma gets enough winter and spring rainfall that many established native plants never need irrigation at all.

For practical decision-making, the cumulative precipitation line is more useful than the daily bars. Compare it to the same point last year (when multi-year data is available) and you know whether your season is wetter or drier than normal. A dry spring means drought stress arrives earlier in summer. A wet spring means your disease pressure window extends longer into June.

Snowfall appears as a separate metric in winter months. In the lowlands, snow events are infrequent but consequential. A heavy wet snow loads broadleaf evergreens and can snap branches on species with weak wood (Callery pear, ornamental cherry). The dashboard separates snow from liquid precipitation so you can see both the frozen and liquid water inputs to your landscape.

Evapotranspiration: The Water Your Landscape Loses

Evapotranspiration (ET) is the combined water lost from your landscape through two pathways: evaporation from the soil surface and transpiration through plant leaves. It is the demand side of the water equation. Precipitation is supply; ET is demand. The balance between them determines whether your plants are gaining or losing water on any given day.

ET is driven by temperature, solar radiation, humidity, and wind speed. A hot, sunny, windy day with low humidity pulls more water out of the landscape than a cool, overcast, still day. In the Puget Sound region, daily ET runs near zero in winter (cold, cloudy, minimal transpiration from dormant plants) and peaks at 0.15 to 0.25 inches per day in July and August.

Those summer numbers sound small. They are not. At 0.20 inches per day, your landscape loses about 1.4 inches of water per week to the atmosphere. If it does not rain (and in July, it usually does not), that deficit accumulates. An established lawn needs roughly an inch of water per week. A newly planted tree needs more. Without rainfall, all of it comes from irrigation or stored soil moisture.

The ET value on the dashboard is a reference rate, calculated from weather data using standard methodology. Your actual ET depends on what is growing: a mature tree with a full canopy transpires more than a bark-mulched bed, which transpires more than bare soil. The reference rate gives you the baseline. If your landscape is mostly turf and mixed plantings, actual ET is close to the reference. If it is mostly mulched beds with young trees, actual ET is lower.

Water Balance: When to Start Irrigating

The water balance chart is the most actionable visualization on the dashboard. It plots cumulative precipitation against cumulative ET since January 1. As long as the precipitation line runs above the ET line, your landscape has a water surplus: more water going in than coming out. When the lines cross and ET moves above precipitation, you have entered deficit. The landscape is losing water faster than rainfall replaces it.

In a typical year here, the crossover happens sometime in May or June. That is the date your irrigation season begins, whether the calendar says so or not. In a dry spring, it can happen in April. In a wet year, it might not happen until July. The chart below shows you the actual crossover for each station, which is a far better irrigation trigger than any fixed calendar date.

The gap between the two lines after crossover represents your cumulative water deficit: how much more water has left the landscape than has arrived as rain. That deficit is what your irrigation needs to replace. By late August, the cumulative deficit in the Puget Sound lowlands typically ranges from 8 to 14 inches, depending on the station and the year.

The station comparison makes the regional variation visible. Sequim enters deficit earliest because it receives less rain and more sunshine. Olympia and Tacoma hold their surplus longer because they receive more spring precipitation. Seattle's urban heat island pushes ET slightly higher, accelerating the crossover. If you are scheduling irrigation for a property portfolio across the region, the water balance chart tells you which sites need water first.

For the home gardener, the rule is simple: when the lines cross for your nearest station, start watering your newly planted trees and any plants that are not fully established. Mature native plants with deep root systems can draw on stored soil moisture longer. But anything planted in the last two years is living on what the top 18 inches of soil can hold, and once the balance goes negative, those reserves draw down fast.

Dew Point: The Disease Predictor

Dew point is the temperature at which the air becomes saturated and moisture condenses on surfaces. It is the single best weather metric for predicting leaf wetness, and leaf wetness is what drives the majority of fungal disease infections in managed landscapes.

The mechanism is straightforward. When the actual air temperature drops to the dew point, condensation forms on leaf surfaces: dew, fog, or a thin moisture film you may not even notice. Fungal spores that have landed on the leaf germinate in that moisture. The longer the leaf stays wet, the more likely infection succeeds. A leaf that stays wet for four hours overnight might escape infection. A leaf that stays wet for twelve hours almost certainly will not.

The chart below shows daily average dew point for each station. The disease risk threshold band sits at 55°F. When dew point averages above 55°F and air temperatures are in the 60 to 80°F range, conditions actively favor fungal pathogens. Apple scab, black spot on roses, anthracnose on dogwoods and maples, powdery mildew on a dozen ornamental species: all of them thrive in that window.

The disease calendar here is shaped by dew point patterns. Spring starts wet and cool (high relative humidity, moderate dew points). As the season warms, dew points rise into the risk zone right as air temperatures create the warm, humid conditions that fungi exploit. The worst disease years are not the wettest years overall. They are the years where late spring delivers a combination of warm days, mild nights, and dew points above 55°F for extended periods. That combination produces the prolonged leaf wetness that pathogens need.

The station comparison on dew point data is subtle but meaningful. Stations near marine water (Tacoma, Seattle) tend to run higher dew points through summer than inland stations. Sequim, behind the rain shadow, runs the lowest. If you are managing disease-prone species like crabapples, roses, or susceptible dogwood cultivars, the dew point pattern for your nearest station tells you how aggressive your fungicide program needs to be. A property in Sequim might get away with one or two preventive applications per season. The same property in Tacoma might need four.

The practical use of the dew point chart: when you see the line climbing toward and staying above 55°F in late spring, your preventive fungicide window has opened. Applications made before the first sustained high-dew-point period are protective. Applications made after the first infection events are reactive and less effective.

Frost Events: Timing, Severity, and What to Protect

Frost is the simplest metric on the dashboard and the one with the most immediate consequences. The frost event timeline shows every night where the minimum air temperature dropped below 32°F, color-coded by severity: light frost (28-32°F), moderate frost (24-28°F), and hard frost (below 24°F).

In the Puget Sound lowlands, the frost season typically runs from late October through mid-April, but the dates that matter are at the edges. A frost in November damages little because most plants are dormant or hardened off. A frost in early April, after warm weather has pushed buds to open and new growth to emerge, can destroy an entire fruit crop in a single night.

The dashboard frost data is measured at standard weather station height (roughly 5 feet above ground). Actual conditions at ground level and in low spots are typically 3 to 5 degrees colder. If the station records 33°F, your garden in a low-lying area likely experienced frost even though the station did not register one. If the station records 28°F, ground-level temperatures in frost pockets may have dropped to 23°F, which is hard freeze territory and damaging to most actively growing tissue.

The station comparison shows the frost geography of the region. Sequim and Kent log the most frost events: Sequim because clear skies allow maximum radiative cooling, Kent because the Green River valley collects cold air that pools on the valley floor. Seattle logs the fewest, insulated by both urban heat and proximity to Puget Sound's thermal mass.

The severity distinction matters. Light frost (28-32°F) damages tender annuals, emerging flower buds, and soft new growth. Moderate frost (24-28°F) kills most actively growing herbaceous tissue and can damage hardwood flower buds that have begun to swell. Hard frost (below 24°F) causes structural damage: bark splitting on thin-barked species, vascular injury in young trees, and death of any tissue not fully hardened.

If you are growing fruit trees, the frost timeline combined with the GDD chart tells you the critical story: has warmth pushed the buds far enough along that the next frost event will catch them at a vulnerable stage? A frost at 500 GDD₃₂ hits mostly dormant tissue. A frost at 800 GDD₃₂ hits tissue that may be at or past bud break. The combination of the two charts gives you the risk picture that neither one provides alone.

How the Metrics Work Together

Water and disease metrics form an interconnected system, and reading them together is more powerful than reading any one in isolation.

Early spring in a typical year: precipitation is high, ET is low, the water balance shows a large surplus, and dew points are moderate. Disease risk is present (it is wet) but limited by cool temperatures. This is when early-season infections of anthracnose and leaf spot establish, but progress slowly because conditions are cool.

Late spring: precipitation tapers, ET rises, the water balance surplus shrinks, and dew points climb into the risk zone. This is the peak disease window. Warm, humid conditions favor rapid fungal development, and the residual moisture from spring keeps leaf wetness duration high. Preventive fungicide applications need to be in place before this window opens.

Summer: precipitation drops to near zero, ET peaks, the water balance goes solidly negative, and dew points may remain high near the coast or drop in rain shadow areas. Disease pressure shifts from foliar pathogens (which need leaf wetness) to root diseases and drought stress. The irrigation need is at its maximum, and the gap on the water balance chart tells you how much.

Fall: the first rains return, ET drops, the water balance begins to recover, and frost events appear. The disease focus shifts to protecting woody tissue: copper applications on stone fruits and ornamental pears before fall rains drive bacterial infections into dormant buds.

A late frost event in spring, plotted on the same timeline as a rising dew point, tells you to watch for Pseudomonas blossom blast on ornamental pears, which exploits exactly the combination of frost-damaged tissue and wet conditions. The frost chart says the damage occurred. The dew point chart says the pathogen has favorable conditions. Together, they are the early warning system.

Getting Started

Check the water balance chart once a week from April through October. When the lines cross, start irrigating anything that is not fully established. That single habit, based on one chart, will prevent more plant loss than any other action in a dry-summer climate.

If you manage disease-prone plants (roses, crabapples, dogwoods, stone fruits), add the dew point chart to your weekly check. When it crosses 55°F and stays there, your fungicide window is open. Applications made in that window are preventive. Applications made a month later are damage control.

For frost, set a reminder to check the dashboard on nights when the forecast calls for clear skies and calm wind from October through April. Radiative frost forms under exactly those conditions, and the dashboard's historical frost data shows you how many times it has happened this season and how severe the events were.

The water is moving through your landscape every day, whether you measure it or not. These five metrics make the movement visible, and once you can see it, you can manage it.

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This guide supports the weather dashboard at /weather/ and is the companion to the heat and growth metrics guide. All metrics are tracked daily from six Puget Sound stations using Open-Meteo historical and forecast data. ET values are reference evapotranspiration calculated from temperature, radiation, humidity, and wind data. Dew point disease thresholds are general guidelines; individual pathogens vary in their moisture and temperature requirements.

Sources: HortGuide 6-station weather network (Open-Meteo API); WSU Extension irrigation scheduling guidance; Cornell University plant disease diagnostic resources; UC Davis reference evapotranspiration methodology; NC State Extension plant pathology fact sheets; HortGuide field observations, Kent, WA, 2025-2026 season.