Ivy League Climber

Wrigley Field is the venerable 1914 baseball stadium on Chicago’s north side. One of its most characteristic features is the ivy covered outfield wall that occasionally swallows a hit ball, never to be seen again – a ground rule double.

The question of the day:

English ivy (Hedera helix) is of the Araliacae family, but doesn’t have spines like some other species in the family, like the aptly named devil’s walking stick (Aralia spinosa). I can’t imagine a Chicago Cub outfielder diving into a wall covered with Devil’s walking stick to make a catch; although being a Cubs fan can feel like that.

English ivy is an evergreen climbing vine, but it will grow along the ground just fine if there is nothing available to climb. Not unlike Kudzu in the US south, ivy can become invasive and choke out other plants, creating “ivy deserts.”

As English ivy grows along the ground, it shows its juvenile form. It has light colored leaves with lobes and points, no flowers, and can form roots very easily. When the ivy finds something on which to grow vertically, it transitions to the adult stage, with leaves that are less lobed or pointy, less root growth and can produce flowers and berries.

The stem of ivy is not capable of supporting the weight of the vine – it can’t stand up on its own, but yet it easily grows 30 m (98 ft) against the force of gravity and can reach heights of more than 90 m (300 ft) in conifer trees with seemingly no problem whatsoever. The mechanism by which it accomplishes this was investigated by none other than Charles Darwin, but much more recent work is showing the ivy plant to be quite a surprise.

Darwinnoted that ivy sends out adventitious roots from its stem. This is where the devil’s club or walking stick and the ivy are similar, but in the case of ivy, they are induced by a vertical substrate and don’t cause pain.

Adventitious roots are those that arise from someplace other there where you would expect them, like directly from the sides of stems, or off leaves, or off old woody roots. In the case of ivy, they are aerial, adventitious roots, since they do not get buried in dirt. They can still collect water, but are protected from dehydration by having a thicker, waxier surface.

Darwin also noted that ivy was not wrapping the adventitious roots around some protruberance on the vertical surface to allow the vine to cling. Those that do wrap around and grab are called tendril climbers, andinclude clematis, grapes, and sweet peas. In some cases, the clinging apparatus will have only that function, in other plants they will grasp, but can leaf or fruit as well.

Other vines use their stems to wrap around a vertical substrate, the stem twiners and tendril climbers are both examples of thigmotropism (thigmo = to touch, and trope = turn). Interestingly, honeysuckle always coils clockwise while wisteria always turns counterclockwise.

English ivy doesn’t twine, it doesn’t tendril wrap, and it doesn’t burrow into a flat surface to gain an anchor, although it will exploit a crack and grow through or along it. Neither does it just grow up until it touches something and then use its growth to ramble through and around the substrate. Climbing rose is an example of a rambler, it will use its hook shaped thorns to help it stand up as it grows through and around another plant.

No, English ivy uses a chemical adhesive secreted by it adventitious rootlet ends in order to stick to a vertical surface – it can even cling to something as smooth as glass. The secretion is yellowish and forms circular dots on the vertical surface. It is very sticky, and becomes stickier as it dehydrates.

The compound contains polysaccharides that act as a carrying agent for discrete nanoparticles (70 nm diameter) that are responsible for the adhesion to the wall. Amazingly, the way ivy clings to a wall is very similar to how a gecko walks up a wall or hang upside down.

The nanoparticles are like the nanohairs on a gecko’s (or fly’s) foot. They increase the surface area of the material greatly and are so small that they can make very intimate contact with the surface. They get so close to one other that they can use van der Waal’s forces on the atomic level to attract one surface to the other. Studies from 2010 showed that the interactions of the nanoparticles in the yellowish ivy secretion were enough to create the bond, and mimics using polystyrene nanoparticles have become excellent adhesives.

But the amazing abilities of the ivy nanoparticles don’t stop there. They seem to disperse and absorb light energy much better than the metal nanoparticles that we currently use in our sunscreens. Titanium oxide and zinc oxide are the current state of the art in terms of reflecting, dispersing and absorbing ultraviolet rays, but it seems that ivy nanoparticles are 70X better at these jobs than are the metal oxide particles. Our next generation of sunblock may come from ivy – talk about green technologies!

Ivy can help with sun damage in another way as well. By covering the walls of a building, ivy keeps the heat in during the winter by acting as insulation and reflects the sunlight away in the summer, keeping the building warmer or cooler as the case may be. Ivy also deflects much of the rain from getting to the surface that is covered, so it can protect against acid rain damage or other weathering.

But ivy can do damage as well. Any surface that has gaps, like shutters against a wall or wood siding will allow ivy to grow in the cracks and pull them from the wall over time.  It may not create holes in mortar or brick, but it will grow into them and then expand when the stem fills with water. This hydraulic action can break down stone over time and bring a building down if given enough time and opportunity.

The mass of an ivy vine can also cause damage. It can cover an entire plant and keep it from getting enough sunlight to live, but it can also make it top heavy and cause it to fall in a strong wind. I have wondered about this in terms of ivy growing on a building. How much weight does it add to the wall, and would it ever be enough to pull the wall down?

Look atan ivy-covered wall. How much must all that vine weigh? Forestry workers pulling ivy off of conifers say it is not unusual for there to be over 2000 lb.s (907 kg) of ivy on a single tree.

I have been wondering how to estimate the mass of ivy that is clinging to a wall. You might estimate the square footage covered, then cut out one square foot and find its mass, and then do the math to find the total. But if you cut from the bottom, then everything above it will die – not the best experimental design. If you cut from the top or edge, the vine will be immature and have less mass per sq/ft than the average along the entire wall.