Whether it’s outdoors or in a office, plants wilt in most situations because they’ve simply run out of available water. Water is constantly moving in a plant from the soil, into the roots up through the stems and out to the leaves.
Once water enters the leaves, most of the water (typically 95% or more) escapes through the leaves into the air through tiny holes in the leaves called stomata. This process is called transpiration and is somewhat similar to how we as people sweat.
You might think – is this 95% of water just wasted? Possibly, but it’s actually necessary.
If you think about tall trees over a 100 feet tall – water must be delivered to leaves at the top of the tree all of the time. If not, those leaves will wilt and die. When water escapes through the leaves into the air through transpiration, it provides a pulling force. This pulling force pulls water up and out a plant through tiny little tubes called the xylem.
These tubes (xylem) can be thought of as pipes inside the plant that deliver water to all parts of the plant. They are highly effective at stacking up water molecules into long chains and pulling them upward and outward to the leaves of the plant. Xylem exists in all parts of the plant – roots, stems, leaves and everywhere in between.
As water is pulled up the plant though the xylem, the water molecules are all tied together like a long chain (capillary forces).
The pulling force created by transpiration pulls these long chains of water upward and outward to the leaves.
These chains of water create turgidity (meaning the plant is rigid, strong and upright; essentially the opposite of wilting). Plants do not have bones to keep them upright – they rely on this turgidity to keep them upright and strong.
When the soil of a plant runs too low of available water, the water chains in the xylem become thinner and thinner due to less water.
Effectively, the plant is losing water faster than it is absorbing it. When this happens, the plant loses its turgidity and begins to wilt.
Low soil moisture is often the reason this happens but other factors play a role. When temperatures are high and it is warm or hot, the plant loses more water through transpiration causing the plant to wilt if the water needs are higher than what is available.
Likewise, if the air is especially dry, wilting can occur quickly. Plants with higher water needs will wilt much faster than plants with low water needs like cacti and other succulents. For example, Spathiphyllum (Peace Lily) are notorious for wilting very easily but will perk back up quickly when watered.
Occasionally, vascular fungal diseases will clog the xylem tissue and cause wilting. Verticillium wilt is a common example and is often seen in tomatoes and trees.
How To Keep Plants from Wilting
The most practical way of overcoming plant wilt is to provide adequate soil moisture and consider the plant type and environmental conditions it is in (hot, cold, etc.).
For outdoor plants and trees, it’s important to remember that they continue to consume water during winter so it’s important to thoroughly water these plants in fall before winter sets in and the ground freezes, particularly for evergreens and newly installed plants. People will sometimes spray anti-desiccants (compounds that reduce dehydration and prevent drying) on trees (particularly evergreens) during winter to reduce water loss from transpiration.
Ironically, extreme over-watering of indoor plants can also cause a plant to wilt in some situations. Why? When soils are completely saturated with water and are devoid of any oxygen, roots fail and are unable to absorb water effectively.
Hopefully this botanical lesson will help you better understand why plants wilt and what you can do to help overcome it.
Green Side Up,
Matt Kostelnick, Senior Horticulturist at Ambius
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