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How Do Plants Grow in Concrete?

Do you know anyone trying to grow a garden? You may hear complaints about their struggle to keep those plants alive. 


Violets and other plants growing in a crack in the sidewalk.
(Photo by Heather Van Zyl)

Plants need the right soil. Too much clay or sand in the soil and some plants won’t grow. They need the right amount of water. Too little and the plants dry up and die. Too much and they rot. They need the right amount of sunlight. Too much sun can make some plants wilt and die. But provide too much shade and other plants won’t get enough sunlight for photosynthesis, and they won’t survive either. Wow! Plants have a tough life! Or do they? 


Walk through your neighborhood and you’ll likely find plants growing right out of the sidewalk — concrete instead of the perfect soil. No one visits with a watering can; they just get whatever rain falls from the sky. In fact, not only do people not care for them, they walk and wheel right over them. But still the plants survive, and even thrive!


How do they do it?


Plants can be found growing through the concrete of sidewalks, driveways and roads, even inside garage and basement edges. But don’t plants need soil and water to grow? Concrete doesn’t contain soil or water, does it? Concrete is solid and strong, isn’t it?

 

Words to know

Hearty: Exhibiting vigorous good health.

Microscopic: So small as to only be visible by microscope.

Photosynthesis: The process by which plants make energy using sunlight.

 

Not completely. If you look closely, there are microscopic cracks running through it. The cracks are not wide enough to affect its strength, though. At least, not at first. Grab a magnifier. Can you find any tiny cracks in the sidewalk?


Tiny weeds find these microscopic cracks. Soon roots emerge. They need to find food and water to feed the plant. The microscopically thin, very sensitive ends of the roots stretch, finding the path of least resistance in their search. They can fit into the microscopic cracks in concrete. They weave through them like a maze, with water and soil as the prize at the end. 


Water finds a way


Carrots just pulled from the ground.
(Photo via Shutterstock)

You know what else can fit into even the tiniest cracks? Water! So roots can find water even in concrete! Imagine a water bottle. If the lid is on, but not completely screwed tight, water can still spill out of the narrow gap. Even though you can't see the gap, leaks still happen. Water always finds a way!


As roots absorb water, they continue to grow. Roots grow from their ends, getting thicker as they develop. Think about carrots, an edible root. They are thick at their leaf ends but narrow the farther down into the ground they grow. If you pull a carrot out of the ground, you’ll find thinner and thinner roots spreading out.


As roots thicken, they force things to split around them — dirt and even concrete. As the split widens, even more water can get in, feeding the roots so they grow even more! It works the same way as a hatchet splits wood. The end of a hatchet is thin, making only a thin slice in a stump of wood. But if you continue to chop, the slice opens to the width of the hatchet, eventually splitting the log.


Sidewalks are made with gaps. Brick and concrete driveways also have spaces. Those gaps can collect a bit of dirt, which can be just enough to give a seed its start. 


See for yourself


Dandelions growing in a crack in pavement.
(Photo by Heather Van Zyl)

Go for a walk around the block. Many hearty plants can live their full lifecycle right there in a sidewalk crack. How many different plants can you find? Look for dandelions, plantain, lamb’s quarter, clover and grass. 


These sidewalk plants create their own little ecosystems. Pollinators, like bees, seek nectar and pollen from dandelion blooms. Insects find refuge under leaves and may take a nibble or two. Spiders hunt the hidden insects. Seeds drop nearby and the cycle begins again.


Want to see roots in action? Put a wet paper towel in a clear, zip-top baggie. Add a couple of bean seeds. Tape to a window. Watch the roots emerge. See how the tiny ends grow farther and thicker over time. Try adding an obstacle, like pebbles, and see what the roots will do.

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