November 1, 2026

How do plants get water from the soil to the leaves

Plants get water through their roots, and the leaves do the pulling. Water evaporates out of small pores in each leaf, and that loss draws a continuous column of water up through the stem from the soil. The hotter, drier, and windier the air, the harder that pull, so the weather sets how fast a bed gives up its water.

How roots take in water

Most uptake happens near the root tips, through root hairs. Each hair is a single cell stretched into a fine thread. Together they give the root a large surface touching the soil.

Root cells hold more dissolved sugars and minerals than the soil water around them. Water moves across cell membranes toward the saltier side, so it flows into the root by osmosis. From there it crosses the root's inner layers and enters the xylem, a bundle of hollow tubes that runs up to the leaves.

Roots can only take water that touches them. Water drains downward, clings to soil particles, and spreads sideways slowly. A root zone can run dry while soil a hand's width away stays moist. We go further into that in where water enters a plant.

How water climbs the stem

The leaf drives the whole flow. Its surface carries many small pores called stomata. They open to let carbon dioxide in for photosynthesis, and water vapor escapes through the same openings. Botanists call that loss transpiration.

As water evaporates from cells inside the leaf, those cells draw on the water behind them. Water molecules cling to each other strongly, so the pull passes down an unbroken column through the stem to the roots. The energy for the lift comes from the sun, which evaporates water at the top.

Most of the water a plant takes up leaves again as vapor. A small share goes into photosynthesis and new tissue. Along the way it keeps cells firm, carries nutrients up from the soil, and cools the leaf as it evaporates.

What sets the pace

Transpiration follows the air around the leaf. Four conditions push it up.

• Sun. Light opens the stomata and heats the leaf, which speeds evaporation from inside.
• Heat. Warm air can hold more water vapor, so a hot afternoon draws water out faster.
• Dry air. The wider the gap between the wet leaf interior and the outside air, the faster vapor moves out.
• Wind. Still air keeps a thin humid layer against the leaf. Wind strips that layer away and replaces it with drier air.

Humid, cloudy, still weather slows all four. At night most stomata close and the pull drops. Water in the surrounding soil creeps back toward the roots overnight. That refill lets a plant that drooped in late afternoon stand up again by morning.

The soil loses water on its own too. Sun and wind dry the surface directly, and bare soil loses more than soil under mulch or a full canopy. Add leaf loss to surface loss, and a bed in a hot, windy week dries far faster than the calendar suggests.

What our beds showed this summer

We ran soil probes in four beds in our own garden from mid-July to early September 2026. Moisture in the ground bed ranged from 32% at its driest to 62% at its wettest. Raised bed 3 covered the same 32% to 62% range. Raised bed 2 stayed between 36% and 57%, and raised bed 1 held tighter, from 39% to 54%.

The ground bed hit its low of 32% on August 6. Our hottest day of the year, 97°F, came on August 5. That bed held squash and watermelon, both broad-leaved plants with a lot of leaf surface to lose water through. We can't separate the heat from everything else that week, but the timing fits the mechanism.

Rain resets the curve. July brought 10.89 in of rain in our garden, and the ground bed and raised bed 2 both reached their highs on July 24. Our wettest day, 2.46 in on August 20, came the day before raised bed 3 reached its 62% high. September brought only 1.35 in all month, and raised bed 2 hit its low on September 6 and raised bed 1 followed on September 7.

When the soil can't keep up

A plant short on water closes its stomata to save what it has. Closed pores also block carbon dioxide, so growth slows before any leaf droops. If the shortage continues, cells lose the water pressure that holds them firm, and the plant wilts. Fruit crops feel this early, since flowers and young fruit compete with leaves for the same supply. We cover the thresholds in why plants need water.

What this changes for watering

Water for the weather ahead of you. Early morning works best. The soil is cool, less evaporates from the surface, and the water reaches the roots before the midday pull starts. Our guide to the best time to water your garden goes through the trade-offs.

Check the root zone, since the surface dries first and says little about where the roots sit. A finger pushed to root depth, or a probe placed there, reads the water the plant can reach. Our guide on per-bed soil probe readings covers placement.

A soil reading tells you what the roots can reach. It can't tell you how fast the leaves are losing water that afternoon, so a tomato can droop in hot wind while its bed reads comfortably moist.

Siltwell follows rain and the root zone. The Today screen gives garden-level watering guidance from rain. A forecast of 5 mm or more across today and tomorrow turns the watering guidance on the Today screen to hold. Rain of 5 mm or more in the last three days turns the watering guidance to covered. On Pro, a soil probe mapped to a bed sends a dry alert when the two most recent readings are both below 25%, or both above 60 cb on a Watermark probe. You get at most one dry alert per bed per day. Siltwell tells you which bed is dry and leaves the watering to you.

None of our four probed beds read below 32% in that window, so a 25% alert would have stayed quiet through our hottest week. If your beds run closer to the line, put your first probe in the bed with the most leaf and the least mulch, because that bed loses water fastest on a hot, windy day.