what is it about roadsides that promote growth of certain plants?

I have seen in guidebooks that certain plants are often found in “disturbed places and roadsides.” And I frequently observe stands of wildflowers along a busy road, but not growing just a few feet back. But I can’t figure out exactly what conditions exist in those places that would be so conducive to particular plant growth. I mean, I doubt that plants thrive on exhaust fumes. The only hypothesis I can offer is that some plants benefit from the extra water that flows to the side of a road. But that’s hard to support given the terrain that sometimes lines a road or highway. Any botanists / ecologists who can explain why some plants find such settings to their liking? Thanks. xo, C.

A lot of plants and animals are characteristic of disturbed sites because they are more tolerant of such conditions than others. Soil characteristics, light, water availability, frequency of mowing, etc may just not be conducive to the growth of a lot of plants. Therefore such sites are often colonized by fast-growing weedy species that are able to exploit them. In sites that are better ecologically, slower growing species that may be superior competitors in the long run can take over and shade out or other exclued weedy species.

Also, I’m sure you know this but lots of states have wildflower projects along the roadsides.

But that doesn’t explain why some plants prefer to grow there. The wildflowers the states pick to plant are exactly those tolerant of roadside conditions.

This may not be germane in your instance, but a program I saw recently on PBS spent some time discussing the fact that only along the roadways and fencelines would you find the native grasses and flowers. All other arable land had been reduced to virtual monocultures by farming activities. So in many areas, the wildflowers and such grow along the roadways because that’s the only place left for them to grow. I have noticed in uncultivated (uncultivatible) areas, the plant diversity does not end at the freeway right of way.

Among other things: roadside soils will have more sand (from the period of construction; may be saltier (in the north where roadsalt is used during the winter) or have sand from sanding icy roads; will have more oil residue as all the lubricants and fuels that bounce off vehicles and the tar used in road surface asphalt is washed to the side during rainstorms; may have higher concentrations of sulphur or other emissions from the traffic before it dissipates* (more prevalent in high density populations centers than farm tracks in Montana); and will be more compacted from the original construction equipment plus vehicles pulling off to the side of the road. There is also a steady “rain” of debris that comes off vehicles, much of which shatters and is turned into grit when it hits the ground or is smashed by later traffic, then either blown or washed off the road. This can include bottles (silica), cans (iron, aluminum, tin, etc.), plastic (various carbons), wood–including paints and solvents that were on the wood, paper, and any number of other materials, none of which are noted for their excellent contributions to creating loam.

  • This may be captured by the respiration of the roadside plants or ay be captured in snow and then leach into the soil when the snow melts.

It’s my understanding as well that roadside plants tend to be non-native species which thrive in disturbed soil exactly because the native plants, which are much better adapted to the normal conditions away from the road and generally outcompete invaders in their undisturbed habitat, don’t do well in that mini-environment.

The “natural” habitat of many plants of disturbed sites is river banks, landslides, recently burned areas, etc. Humans have greatly expanded the area available to them by disturbing more stable habitats.

As suggested above, lots of little things, I think. The soil off the side of the road has probably been “disturbed” from its natural state by the construction. I’m not sure that it will be more compacted than that beneath the roadway - or the surrounding area - because there’s not so much call for a contractor to spend effort compacting soil not directly under the roadway.

As may be, the compaction will be different than surrounding soil, and the road shoulder soil may have a slightly different composition: perhaps the contractor had to bring in soil from somewhere else to make final grades.

I sort of recall a striking example of all of this driving with my dad along the Alaskan highway many years back. They had to cut a swath through evergreens to build the highway; the first thing to grow back were deciduous trees (possible because of now-available sunlight?) that turned a bright yellow in the fall.

Certainly, the roadside will not be as compacted as the area beneath the roadbed. However, several things contribute to roadside compaction. First, when laying a roadbed, the construction crew will tend to compact the area to be paved a bit wider than the actual bed as a guarantee that they do not have any soft spots if the paving crew is a bit off. Then, the very act of preparing the roadbed means that equipment is working back and forth over a wider area than the roadbed, itself. Finally, a number of departents of transportation require shoulders to be compacted to some extent away from the edge of the road, so, regardless whether the contractor would choose to expend the effort to compact the soil, he may be required by law to do so. (For example, in Summit County, Ohio, for any disruption of a roadbed to lay utilities or culverts, the county requires that any disturbed soil at a 45° angle from the road edge to a distance of 20 feet must be replaced by premium stone (304s or better) and the stone compacted in lifts of no more than 4 inches at a time. Similar rules guide new construction, although I have not done inspection on new highways, and I suspect that Summit is not rare in its rules.)

You are correct that roadside compaction will not be as firm as roadbed compaction, but I suspect that it will be firmer than the earthworm-worked soil in adjacent fields. (The compaction together with the various grits and chemicals washing off the road tend to reduce the number of earthworms, as well, which changes the character of what can grow at roadside.)

I would think that the constant vibrations from passing vehicles would compact soil at the roadside, also.

A lot of CO[sub]2[/sub] and water vapor, what’s not to like?

Honestly, I doubt the water vapor from car exhaust raises the humidity of the roadside much, but on a heavily-travelled road in a dry windless area, it might be a factor.

I’m not totally convinced, somehow. If the CO2 and the H2O are increased at the side of the road, then other plants should profit and grow there, too. I’m still curious as to what it is, specifically, about disturbed areas that would be better for some plants to grow. What IS a disturbed area, anyway, and what factors would enhance plant growth? As to the roadside question, I have seen stands of all manner of plants there. All of them grow elsewhere, as well, but sometimes appear to be “trying” to grow as close to the road as possible. Chicory, Queen Anne’s Lace, Butter and Eggs, Bird’s foot trefoil, other clovers, often appear in those spots. Maybe, as Tomndebb suggest, the salts and oils are supportive of their survival. Hard to believe, but clearly possible. I’ve also seen many of these plants lining walk paths, bringing me back to the question of “disturbed areas.” Something should be simpler here, or more obvious. But it escapes me.

You seem to be overlooking my first post. It’s not that the roadside is better for certain plants, it’s that it’s bad for most plants. This allows certain plants that are better able to cope with what are generally negative conditions for plant growth to dominate, because other plants are unable to grow there very well. You don’t see these plants growing elsewhere because in non-disturbed environments because other plants are able to out-compete them. But roadsides aren’t necessarily the best environment for roadside plants themselves. If you grew chicory in a nice fertile well-watered spot by itself it would probably grow even better than it does by a roadside. But under natural conditions chicory is excluded from such sites by competition, so you mostly only see it near roadsides.

This is an elementary part of plant community ecology. In forest succession, plants tolerant of disturbed conditions, known as pioneers, appear first and are eventually replaced by others. But because of mowing and other continued disturbances, roadside vegetation is generally kept in a stage of early succession by human interference, creating the characteristic roadside community that you see.

Why do you see certain businesses in the seedy part of town? Is it because that part of town is better for those businesses? Not necessarily - it’s just that they are shut out of the better part of town by more competitive and better run operations. Roadsides are the slums of the plant world.

Well, they do get more water, because it runs off the road bed; when we stop watering our lawn, the grass by the driveway stays green because of runoff. And also roadsides tend to be full sun.

The DOT usually mows the roadsides, too, so anything bigger than a wildflower won’t live. Maybe Queen Anne’s Lace and clover just happen to be the plants that thrive best in those conditions, which are kept artificially short by those mowing guys?

I just realized I don’t think I’ve ever seen a roadside dandelion, which thrives in similar conditions in lawns. Hmm.

Depends. That may go for areas in which the roadside is below the level of the road, but not where the roadside slants upwards. In any case, compaction may cause greater runoff so the water is less usable to plants.

Dandelions like really short vegetation like well-mowed lawns. Once the grass gets a little higher they begin to be shaded out. Unless the highway department is mowing very frequently you may not see them. Then again, if they are taking that much trouble they may be applying some anti-broadleaf-weed herbicide.

What about birds roosting at the edges of the open areas? They eat the seeds from these plants and I’m sure some will not digest. The birds will migrate over time. The birds expell the seeds along with a bit of “fertilizer” and presto, new plants!

Yes, that’s clearly a significant seed dispersal, zone sowing vector. Clearly, it’s one reason that certain plants grow along fences, under branches and wires, etc. I wonder how it could be connected to disturbed regions - areas that, as Colibri points out - are in the early, and sometimes perpetual, stage of succession.

This is not a general explanation for the occurrence of certain plants by roadsides, since birds disperse seeds in all habitats. But seed dispersal is particulary important for pioneers, as I discuss below.

Pioneers/early successional species must have good means of dispersal, since areas of suitable habitat normally don’t persist very long and they must find newly disturbed ones. (As plants grow on a disturbed site, it changes soil and light conditions so that the pioneers are shut out, which is the process that results in succession.) In the temperate zone, the majority of roadside plants have wind-dispersed seeds (e.g. asters, thistles). If they are dispersed by animals, it’s often in the form of sticky seeds (burdocks, sticktights, etc.) that cling to the fur or feathers of animals (or to pants legs). There are relatively few that produce fruits suitable for birds (e.g. blackberries). The situation is different in the tropics, where many pioneer species have bird-dispersed fruits.