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I have been told that debris/pollution from the road will find its way into my plants, but I don't know how much to be concerned about this.

We have a nice porch which could be used as a vegetable garden, but it is near a side street. It is, however, elevated from the street by a good 10-15'. Should I be concerned about toxins in my food?

What about a planter that is much closer? My neighbors have a vegetable garden that is set back about 20' from a major road (45 mph), at ground level. Should they be concerned?

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Also consider the probability of drive-by veggie stealing. My parents used to have a garden near the road when I was a kid, and this actually happened quite a bit. –  Doresoom Jun 8 '11 at 20:52
@Doresoom Ugh! sounds awful. Our porch is not very visible from the street, though, so it'd have to be an 'inside job'. ;) –  Alex Feinman Jun 8 '11 at 23:57
@AlexFeinman In the UK if you look where a lot! of Allotments (Community Gardens in the US) are located, they are not in the "best" locations eg Right next to railway tracks... –  Mike Perry Sep 29 '11 at 15:19

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The primary concern has historically been lead poisoning, but leaded gas started getting phased out in the US in 1973 and was finally banned in 1995. If your garden is in an older city neighborhood, then it is possible that you've got significant lead levels already.

We have a nice porch which could be used as a vegetable garden, but it is near a side street. It is, however, elevated from the street by a good 10-15'. Should I be concerned about toxins in my food?

The elevation makes me feel that it is safe enough for food production. Actual street-side would be something for flowers in my opinion. I've got neighbors who plant next to neighborhood roads with significant traffic (just on the other side of the sidewalk from the street), and they've been doing it for 20+ years. They've felt no concerns about pollution or toxins when I've asked them.

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Right, I've seen lots of community gardens wedged in between train tracks and city streets. But then again, I don't know how many of these folks die of cancer. –  Peter Turner Jun 8 '11 at 19:09
@Peter Compared to their peers, I bet they live longer because the benefits of fresh fruit and veg almost certainly outweigh minor, slow poisoning from air pollution. –  winwaed Sep 29 '11 at 13:49

It sounds like you would be safe from runoff being elevated that much above the street, but depending on the traffic level, you could still be getting fumes and such from passing cars. I would personally plant there, but if you're leery of it, I would get your soil tested beforehand.

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+1 For getting soil tested, peace of mind... But not needed when planting in pot/containers (if potting medium is new/fresh), though could be worthwhile just before harvest time to see if the "soil" had taken in anything unwanted... –  Mike Perry Sep 29 '11 at 15:15

Another thing to be wary of would be salt if you are in a northern climate where the roads are gritted/salted.

Yes there might be toxins to be wary of - but you breathe those in when you're driving a car anyway. I.e. the risk would be small compared to other risks.

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One of the more permanent solutions to this very problem is to build a berm at the edge of your property. Doesn't have to be big, just enough to segregate surface water during the heaviest rains.

A berm keeps roadside water roadside, and also helps trap water that falls on your property and keep it there rather than running off into the storm sewers. The roadside water will contaminate the roadside half of the berm, but much of the contamination won't travel horizontally through the soil because rain water drives it all downward. Of course once it hits the water table, then it's going to spread, but that's going to happen no matter what. This at least keeps your surface soil and water segregated from street water.

It's a common component of permaculture.

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I know where I from originally in the South Carolina sand hills, the Department of Transportation regularly sprays herbicide on the sides of the roads via a slow moving truck in order to stay the cost of roadside tree trimming. It is done more so in rural areas along dirt roads but if I were you, I would check to make sure it's not done in your area.

See Reference.

"EVistas EVistas - Weeds Meet Their Match in South Carolina." EVistas. , n.d. Web. 23 Mar. 2015.

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There are a number of problems associated with growing edible plants near the road, as well as near cement, driveways and all that.

Fly ash is an important ingredient of most cement, as well as other things (including potentially the road). Fly ash contains heavy metals. Plants could absorb these heavy metals.

Here's what I would do: Put some kind of lining in the ground between the road and your soil to prevent your plants' roots from touching the substance containing fly ash. Never put cement, concrete gravel and stuff like that in your soil (like if you're shoveling snow and bits of the driveway come off with it, don't throw it on your lawn, as you may want to garden there some day).

Your neighbors and others happening by might eat or play with your fruit.

You never know what's in car exhaust.

All in all, if there weren't too much traffic, I would still do it if I were running low on places to plant, but I would put that lining between the soil and the road. This may also keep your tree roots off the road. If there were a load of traffic, I'm not sure what to say, although from what I've seen exhaust doesn't look terrible for plants in ways that would harm humans eating them, these days (although lead in the past was a problem). I could be wrong. You sometimes don't know what new changes come about until after the fact.

I wouldn't be that concerned about exhaust with the road being 20+ feet away, but that's me (and this isn't professional advice). However, you could always put a giant hedge close to the road to stop much of the pollution from getting into your yard, if it turns out to be a problem.

Edit: Michael's answer reminded me of the mosquito trucks. They spray stuff all over the roads to kill mosquitos. I'm not sure how far from the roads the stuff gets. Depending on which chemicals they're using and what you want in your garden (including things like insects, fish ponds, and toads), this may be a problem. One of the chemicals they are known to use is not good for amphibians and fish, and that chemical and others may or may not harm beneficial insects. The health impact on humans, I don't know. But I wouldn't ride a bike behind one of those trucks like a lot of kids used to do probably before my time. I wouldn't even be outdoors.

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Yeah, I like to stay at least 10-15' from less busy roads, and farther away from busy ones, if they're old. Often, a road has been in place since cars used leaded petrol, and the ground in the surrounding area often contains lead from the exhaust. The older and busier the road, the worse. –  J. Musser Feb 22 at 3:36

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