So, plant roots are amazing things! To us they might seem feeble and that you can pull them apart easily. But actually they are very, very highly evolved. Roots can grow through concrete in several ways. Firstly concrete is not actually as solid as it might appear! There are lots of small cracks that your eye might not be able to see but which are perfect for roots to start to push themselves into. Once they have found a weakness, they use a huge pressure in their meristem cells to force themselves into the crack, further separating the concrete. Pressure of about 15 bar are about the limit of most plant cells. Plant cells have a cell wall which means that they can reach much higher pressure than animal cells which would just explode. They also can produce chemicals in their root cells which can dissolve concrete, probably types of acids! Also as concrete ages it cracks and warps too – as it freezes and thaws for example and roots will only too quickly exploit these weaknesses.