Knotweed and trees...

[Gary Prentice]

Sure you're right:001_smile: it's just that surely Tarmac is(or is meant to be) impermeable to water otherwise every frost it would disintergrate.

 

You have a point there. I know there are no-fines specifications for permeable tarmac, but don't recollect any mention of the effects of frost. One of the problems is that a lot of the research is from the USA, with their broad climate extremes.

 

I think mature trees would be more tolerant of tarmac laid right to the root flare, due to the extent of the spread of the root system. (often far beyond the drip line dependent on soil conditions).

 

I'll try to research an answer as your question is bugging me now:confused1:

[Gary Prentice]

No mention of frost heave, but some notes on the permeability of non porous tarmac/asphalt.

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/7728/pavingfrontgardens.pdf

[Le Sanglier]

porous Tarmac exists but the in the ordinary urban setting it's the non porous sort. Another question from a Neanderthal, isn't there some sort of need for the roots (at least the little ones near the surface) to access air? How are they managing that under all that various aggregate and impermeable hardtop?

Ps thanks for the link, I read what I could. Got to get going, so apologies if I missed something in there,

Edited October 9, 2013 by Le Sanglier

[Gary Prentice]

I don't think that any tarmac is totally impermeable from what I've briefly read this morning, the greater the percentage of fine particle sizes the less permeable it becomes. Water will slowly drain through.

 

Under tarmac, due to I think the water present due to condensation, fine roots are normally present, even growing into the tarmac itself. Therefore there must be enough air present. I think the oxygen levels aren't that high, 10-15% off the top of my head, for root growth. Air and water will also diffuse laterally through the strata, particularly in the sub-bases of compacted stone (lots of micro and macro pores).

 

Its a whole new subject for me and I'm still reading but some of it's heavy going and like most subjects there's so many factors involved. I'm reading James Urban's "Up by Roots" and the ISA's series of "the Landscape Below Ground" at the moment. Along with various books on soils etc.

[Le Sanglier]

I guess it must be permeable otherwise they'd die! Still, the amount of water that reaches the roots must be minuscule compared to a tree in normal surroundings.

[Taz Taylor]

Thanks for the replys, the membrane is a permeable one. Stem injection wasn't used and they are trying to cram as much as they can into the area and is being tarmac'd and used as car park spaces and walkways...

 

Like you said, trees along roadways etc are tarmac'd up to seem to still thrive but with another form of barrier going in as well could it possible do more damage to the trees then good.

 

The more I think about it, Tarmac, whacker plates and rollers around trees can't be good.

[Gary Prentice]

Thanks for the replys, the membrane is a permeable one. Stem injection wasn't used and they are trying to cram as much as they can into the area and is being tarmac'd and used as car park spaces and walkways...

 

Like you said, trees along roadways etc are tarmac'd up to seem to still thrive but with another form of barrier going in as well could it possible do more damage to the trees then good.

 

The more I think about it, Tarmac, whacker plates and rollers around trees can't be good.

 

Car parking under mature trees:thumbdown: presumably this hasn't gone before the LPA then?

[Gary Prentice]

I guess it must be permeable otherwise they'd die! Still, the amount of water that reaches the roots must be minuscule compared to a tree in normal surroundings.

 

Hence the life expectancy of new urban tree planting and lower growth rates. It's an extremely hostile environment, poor compacted/contaminated soils, hydological problems, limited gaseous exchange, enhanced temperature and wind speeds raising transpiration rates and us with chainsaws. It's a wonder any survive.