October 13, 2026

Why does a plant need water?

A plant needs water for four jobs at once. Water inflates its cells so stems and leaves stand up, supplies the photosynthesis that makes its sugar, carries dissolved nutrients up from the roots, and cools the leaves as it evaporates. As soil dries, these jobs fail in a rough order. Growth slows first, while the plant still looks fine. Wilting comes later, so a drooping leaf is a late signal.

The four jobs water does

Most of a soft leaf or stem is water. Each job below draws on the same supply, which is why a dry bed touches all of them.

• Turgor. Each cell holds a central sac of water, the vacuole, that presses outward against the cell wall. That pressure keeps soft stems upright and leaves spread flat to the light.
• Photosynthesis. The leaf splits water molecules to power sugar-making, and the oxygen it releases comes from that water. Carbon dioxide enters through pores called stomata, and water vapor leaves through the same pores.
• Transport. Water moving up the xylem carries nitrogen, potassium and calcium from the roots. Calcium travels almost only in this stream and stays put once it lands in a leaf or fruit.
• Cooling. Evaporation from the leaf takes heat with it. A sunlit leaf with open stomata runs cooler than one with closed stomata.

How water gets from soil to leaf

Water evaporates out of the leaf through open stomata. That loss pulls on the column of water in the xylem, which runs unbroken from leaf to root. The pull draws more water in through the root hairs, which take it from the thin film clinging to soil particles. The sun's heat drives the evaporation, so the plant spends no energy on the lift. Our guide on where water enters a plant covers which part of the root zone does the drinking.

Which job fails first as soil dries

Wet soil holds water loosely in its large pores. As the soil drains and roots draw it down, the water that remains clings to particle surfaces and tiny pores. Roots must pull harder for each sip. Gardeners measure that pull as soil tension, in centibars.

Cell growth is the most sensitive job. A new cell enlarges only while turgor pushes its wall outward, so even mild tension slows expansion. Leaves come in smaller and fruit sizes up slowly, with no droop to warn you.

Next the plant closes its stomata to stop losing water. That saves water and also shuts carbon dioxide out, so photosynthesis drops. The flow up the xylem slows, and calcium delivery to fast-growing fruit slows with it. In tomatoes and peppers, fruit that formed during a dry spell can show blossom end rot later, because its tip got too little calcium while it grew. The leaves also lose their evaporative cooling, and on a hot day a closed leaf heats above the air around it.

Visible wilting comes after that, when cells lose enough pressure that tissue goes limp. Past the permanent wilting point the roots can't pull water from the soil at all. A plant there won't recover overnight, even in cool air.

Why leaves wilt on hot afternoons

Squash and cucumber leaves often droop in mid-afternoon in soil that's still moist. Their large leaves lose water faster than the roots can replace it in peak heat, so turgor drops for a few hours. By evening, uptake catches up and the leaves lift. A plant that's limp in the cool of early morning is short on water. A plant that's limp only in the afternoon heat may be fine, so check the soil before you water.

What our beds showed this summer

We have soil probes in three raised beds and one ground bed. From mid-July to early September, raised bed 1 ran 39% to 54% and raised bed 2 ran 36% to 57%. Raised bed 3 and the ground bed each swung from 32% to 62%.

The ground bed is 8 inches deep, against 24 inches for the raised beds. It hit its low of 32% on August 6, the day after our hottest day of the year reached 97°F. Watermelon, red kuri squash and celery were growing there, and big leaves in that heat move a lot of water. A shallow bed has less water in reserve to cover the loss. July brought 10.89 inches of rain and August 6.56 inches, which drove the highs. None of our probed beds dropped to 25% in that window.

What a dry bed reading tells you

A capacitive probe estimates how much of the soil volume is water, as a percent, from how well the soil stores an electric charge. A Watermark sensor reads tension in centibars, which tracks the pull roots feel. A higher centibar number reads drier soil.

Siltwell checks each probe mapped to a bed every 10 minutes. It raises a dry alert for that bed, with a push notification, when the two most recent readings are both below 25% on a capacitive probe or both above 60 cb on a Watermark. Requiring two readings in a row keeps one odd reading from firing the alert. You get at most one push per bed per day, and every bed uses the same threshold. Per-bed alerts need Pro and a connected station with a probe mapped to the bed. The alert tells you which bed is dry and leaves the watering to you.

Suppose your bed reads 26%, then 24%, then 23% over three checks. The alert fires on the third check, once two readings in a row sit below 25%.

The percent on a capacitive probe doesn't account for your soil's texture. Clay holds water so tightly that 25% can sit close to the point where roots stop getting any, while 25% in a sandy mix is still wet. We can't tell you where your own soil's wilting point falls on the sensor's scale without testing it, and our guide on soil moisture content versus sensor percent shows how to check.

What to do with a dry reading

Water the bed that's dry and leave the others. Soak it deep enough to reach the roots. Frequent shallow sprinkles wet the surface and leave the lower root zone pulling against dry soil. Our guide on soil moisture and when to water walks through timing.

After you water, watch how high each bed's reading climbs and how fast it falls. That rise and fall is the bed's baseline. A bed that tops out lower or drops faster than the rest will need water sooner. In our garden, the shallow ground bed and raised bed 3 swung the widest. Our guide to finding a per-bed baseline shows how to read those curves.