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  Paul McNulty June 14, 2012 at 10:20pm

  I wouldn't use grass clippings as a mulch, let them rot down a bit first. They tend to make a solid, soggy mass that's going to stop air getting to the base of the plants.

  As for feeding Lleylandii? How fast do you want them to grow, lol!
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  Dan Frazer Gardening June 14, 2012 at 10:26pm

  Paul I just need them established. They are planted 30m away from the house to form a wind barrier, so the bigger and faster the better. Even at full size (whatever that may be) they will not be too big for this position.

  Would a thick compost mulch possibly help? I'm not sure if they would actually benefit from it in the end.
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  Brian www.mibservices.co.uk June 14, 2012 at 10:29pm

  Hi Dan, its not going to do them any harm at the size they are, its only going to introduce some extra nitrogen into them. When planting leylandii, i have always advised for trees no more than a metre in size. Larger Leylandii specimens always seem to struggle to establish themselves once unpotted and planted. Younger, smaller Leylandii will very soon catch up on a 6-8 foot container grown specimen planted. 4 feet a year for the first few years. 6-8 foot trees take longer to root out. Problem could also be over composting, leylandii will grow in any soil type so their need for nutrients is minimal.
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  Dan Frazer Gardening June 14, 2012 at 10:33pm

  I think I'll leave them, control the weeds and focus on support from the wind.
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  Colin Hunt June 15, 2012 at 6:28am

  If they had a tightish rootball, we would have stood them in water for a while to make sure the centre is soaked, which helps absobsion for the first few months until the roots spread and find their own supply. They have possibly just semi dried out even if it has been the wettest spring - ever!
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  Tim Haywood June 15, 2012 at 7:05am

  Are they too wet or too dry? The holes could be acting as sumps and flooding or the lot could be too dry. I would remove the weeds and put a mulch down -- any nitrogen robbery only takes place on the thin layer of soil/mulch interface, it won't do any harm as long as it isn't dug in: http://aswcd.org/Woodchips.pdf
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  Chris Auld June 15, 2012 at 7:24am

  I would never use grass clippings as a mulch, they will turn to slime if spread out, grass only turns to compost if stacked in a big pile where the heat will build up. Have you considered woodchip mulch? I have a Leylandii hedge at the side of my garden which im gradually allowing to grow taller so my nosey neighbours stop peering into my garden, ive been feeding it with growmore a few times a year and its working great.
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  Paul McNulty June 15, 2012 at 11:15am

  Woodchip is the worst thing to mulch with, horrible stuff. Sell it for bio-fuel! Fresh grass clippings are not ideal, and if thickly-spread will just be a smelly mess. Whatever you mulch with should be composted properly first, and of course then it's a perfect seed-bed, so useless for keeping weeds down. Mulching is about conserving moisture in the soil and feeding the plants slowly. Fresh material robs nitrogen as it rots.
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  Tim Haywood June 15, 2012 at 7:38pm

  Well I don't think her nationality has a great deal to do with things. Maybe the Americans are ahead of us in that regard.

  Here's a more complete text from her scientific paper, rather than the magazine article:

  Nitrogen deficiency. A common misconception about woody mulches is that they impose a nutrient deficiency upon plant materials. This is based on the fact that woody mulches have a high C:N ratio and nitrogen will be ‘tied up’ by microbes during the decomposition process. Furthermore, woody materials that are used as amendments incorporated into soil or potting mixes will create zones of nitrogen deficiency, which is visualized by spindly, chlorotic growth of plants in these zones.
  Experimental research reveals that neither nitrogen immobilization nor growth suppression occurs as a result of using woody materials for mulch. To the contrary, many studies have demonstrated that woody mulch materials actually increase nutrient levels in soils and/or associated plant foliage.
  
  

  Have you discounted other reasons for plant failures in supermarket car parks? Are you sure it's the bark? Have you considered poor (or no) soil preparation by contractors, no subsequent irrigation during establishment etc.

  Duncan said:

  With all respect due, that's not what she says plus

  we are not talking about established plants here.

  She makes no mention of the effect of tannins on plants as far as I can see, after a quick skim read.

  And with all due respect to the author, she's American, they mulch everything in sight.

  My hypothesisis that a zone

  of nitrogen deficiency exists at the

  mulch/soil interface, inhibiting weed seed

  germination while having no influence

  upon establishedplant roots below

  the soil surface.

  It's poses no problems after the plants have been in the ground for a few years, during the establishment phase it does. I could go out and take some photos of the effects today, the evidence is all around us, in almost every supermarket car park.

  
  Tim Haywood said:   any nitrogen robbery only takes place on the thin layer of soil/mulch interface, it won't do any harm as long as it isn't dug in: http://aswcd.org/Woodchips.pdf