Suggest you Google your state or county extension service list of invasive species before introducing Japanese knotweed (Fallopia japonicum; aka, Polygonum cuspidatum Sieb. & Zucc.) anywhere near your property. It is extremely invasive here and nearly impossible to eradicate or remove. It self-seeded in my vegetable garden at my previous house and within a year or two had quadrupled in size. Since it wasn't growing anywhere nearby, it had to have been established from a seed dropped by a bird or carried in by the wind. I tried smothering it along with digging it out but it grew 10 ft. tall and was far too difficult to remove. I'm guessing in the absence of a flame thrower, it's taken over the entire garden by now.

Here's what a Google search turned up:

Japanese Knotweed is spread by seeds and by root fragments that are transported in fill dirt or by moving water. It sprouts out from below ground in early Spring and grows rapidly and profusely on roadsides, streambanks, wetland margins, floodplains, and gravel bars.

Hopefully others will offer suggestions for blocking the view of your yard from the road. One of my garden priorities has been curb appeal so I can't offer advice in that regard. Best of luck!

Wouldn't recommend it :-) You may temporarily limit the rhizomatous spread with some sort of root barrier but it can also be disbursed by seed. Unless you are dedicated to diligently deadheading the plant.......And those roots can penetrate very deeply, as much as 3 meters (10').

There are so many more attractive and much better behaved plants that could accomplish a similar task that is makes little sense to contemplate intentionally growing such a known and perniciously invasive species.

That's interesting because I just read that it does not spread by seed. As far as different plants to acomplish the same thing, I actually posted a pic of the area on garden web and nobody could come up with anything because there are so many mature maple trees and the ground is a mixture of dirt and gravel. I am all about curb appeal this is not the front of my house it's an area about 300 feet long that runs along the side of the house. I am on corner. Well back to the drawing board

Here's a link to Lisa's previous question which includes a photo of part of the stretch of concern. Lisa, can you answer the questions I asked in March and then you may get may get some additional suggestions.

Here is a link that might be useful: Lisa's previous question

Wow, it was nice of you to find that link. I'm sorry, I didn't realize that you had asked me other questions. So, yes, deer can be an issue, although luckly not a huge issue. I do have lots of other bushes in the yard (hollies, oak leaf hydrangeas, etc) but they don't do a good job of providing privacy.

"What am I trying to accomplish" is a good question. When I moved into this house 20 years ago, there was hardly a car on that road. Now, even though we are out in the country, that road seems like a freeway (it's a back route heading into NJ from NY). I can't stand sitting in my nice back yard, and seeing (and hearing)cars go by constantly. Also, the pic that you linked to, I actually found ON LINE by googling my address. Up until then I had no idea that anyone could get a street level view like that. Kind of scarry. I suppose that I could just do the origional 100' that I had said. I don't have to go to the back of the property.

I don't know the PH of the soil, but it's mixed with a lot of gravel for some reason. Also, as you can see, it's very shady. I'm looking for something that will spread, in order to keep costs down.

Do you need privacy year round, or really just in warmer weather? This question is important since it decides whether you are limited to just evergreen plants.

You'll still hear cars with plants; only walls or earth berms will really block sound. If you plant some along the margin of your yard and some along the road, you shoud be able to achieve pretty good visual privacy. Regardless of what you plant, for the first year plan on mulching around the each plant a couple feet out as well as watering them any time you aren't getting regular rain during the growing season.

Is it really 100% shade all day or is there some sun at the beginning or the end of the day? What side of your house faces north in the photo? The quality of the shade will effect how densely plants grow. The list below are plants that will grow in part shade or high shade, though not densely, in my experience.

Check out clethra alnifolia, any of the shrubby dogwoods like the red-twigged dogwoods, or native viburnums (nannyberry, arrowwood, wild raisin, highbush cranberry), many of which may be available inexpensively from your local conservation service, though if not I've provided a link below to the NH state forest nursery which will ship orders in spring. These will be small and may do better if you grow them in a nursery bed or large pot for a year before planting them out. Many of the more common rhododendrons like R. roseum elegans, Olga Mezitt, and PJM can be bought from big box stores in spring and fall in quart or 2 quart pots, and if you are lucky quite inexpensively. I bought some Olga M and PJM a couple of years ago for $8 per pot and when planted 5 feet apart they will grow together. They may not bloom too well in that much shade, but they should grow and are evergreen.

If you are willing to do some propagation yourself, old-fashioned forsythia grows pretty thick and twiggy enough to provide some privacy even in winter. They root and grow fairly easily from cuttings and then grow fairly quickly. If you peg down the lower branches of the rhodies, they will root and then can be transplanted in a year or two.

Some needled evergreens may grow there. We have balsam fir growing in part shade in the woods, though I am not sure if you are too warm for it. White pine may grow for you there, though it won't stay at a height to block cars after about 20 years and won't be super thick growing in shade.

Regardless, start working to get rid of the knotweed now, disposing of the seeds in a trash bag, not as compost. Either cut them down constantly or apply frequent coats of herbicide whenever you see leaves so that this doesn't get out of control.

Here is a link that might be useful: NH State forest nursery

Than you for that link, I will start looking at some of the plants that you suggested. Because of the canopy of the trees, the area gets no direct sun. However, it is not dark shade. I don't care so much about privacy during the winter, just mid-spring to mid-fall.

I think we may be just slightly too warm for balsam fir, as I don't see any in my area, but I do see them about 30 minutes north of us. I believe that the area in the photo is the south west side of the house. The front faces north. Amazing that after 20+ years of gardening, I'm not 100% sure about that.

On the inside of the yard, I have been slowly adding bushes that block the road in various spots. I have lilacs, rose-of-sharron, butterfly bush, a sand cherry, and even some bamboo. The problem is that this method is taking forever. That's why I'd like something that spreads fast on the road side. Will privet or forsythea grow quickly in the shade? I didn't think so, but I might be wrong. Also, do you know which varieties of rhododendrons grow tall, quickly?

lisa2004 - regarding the seeding question. Was your source a US source? I don't know if it sets seed there. But in the UK, where it is a horrendous problem, it still manages to spread despite not setting seed here. The explanation is supposed to be that all JK in the UK is from one parent ie clones and thus all the same sex and unable to produce seed.
Whatever the truth, JK can and will spread without seed since even the tiniest piece of root - ie even a 1/4 inch - can grow into a plant. It could come in in compost, on feet or via water courses.

JK is the only plant I can say I truly loath. The six year three times a year professional herbicide applications it has taken to, almost, remove it from my wood could have something to do with that.

BTW - a flame thrower would not bother JK.

Here is a link that might be useful: JK

You aren't going to find something that spreads that fast. Planting a staggered row of shrubs will give you cover sooner since you would plant the shrubs in the second row alternating with the positions of those in the front row.

With no sun at all, forsythia won't be thick enough. Try white pines or Rhododendrons roseum elegans and PJM, both of which get large and grow relatively quickly after the first couple of years when they are mostly growing roots.

Thanks everyone, I will definately not encourage the growth of the fallopia! I guess the Rhododendrons are the best option, even though I've never been a huge fan of them. It seems that a fence would be my only other alternative. I googled both "elegans" and PJM. The sites I found say that elegans is a moderate grower and PJM is a slow grower. I'll do a little more research before I buy.

If you haven't, check your soil pH before going for rhododendrons. It can vary amazingly in a very short distance north of the terminal moraine, and once you get over 7, rhodos are neither quick nor easy.

Flora, how terrible you had to deal with that monster. it has completely taken over roadsides in NH. Honestly, that's the one invasive that petrifies me and I hope to never see it in my yard/woods.

I was at an invasive talk last year and he mentioned that each "knuckle" along the stem can create a new plant if anyone tries to cut it back, dig it out, etc. The smallest pieces can root. It is very beautiful but a dastardly little bugger.

Lisa, I have found Callicarpa grows pretty quickly in the shade. If you have heavy shade, I'm not sure you'll get as many flowers/berries, but if it likes where it is you'll get seedlings too. I've also found my Kerria japonica spreads pretty readily in fairly deep shade and it does bloom nicely. It sure looks like you'll have a lot of mature tree roots to deal with, so that could be problematic.