[Marilyn]

Hello all,

Was selling seedling Cotoneaster lacteus at our local MGS plant fair on Saturday and had a rather curious comment from one customer about using Cotoneaster near oaks - he had read that this can cause problems for the oaks. Has anyone heard of this and can you explain in a bit more detail? Is it a local thing (UK) or worldwide? Thanks!

[MikeHardman]

I have not heard of this.
Oaks themselves can suppress other species, primarily herbs and grasses, through the release of coumarins as exudates into the soil. Black walnuts suppress many trees, shrubs and herbaceous plants (including cotoneaster) by releasing juglone.

For reference on allelopathy:
- Informative discussion on allelopathy and other issues - http://www.sustland.umn.edu/implement/trees_turf.html.
- Black walnut toxicity specifically - http://www.mortonarb.org/tree-plant-advice/article/887/plants-tolerant-of-black-walnut-toxicity.html.

[Marilyn]

Thanks for that Mike, yes I was surprised by it too. Haven't been able to find anything online so far. As you are, I was already aware of several trees/larger shrubs which discourage growth around them (Rhododendron, Eucalyptus and pine being other obvious additions to this group), but Cotoneaster was not one of them.

[Alisdair]

Marilyn, I'd go further than Mike and say that it's virtually inconceivable that an oak could be damaged by any cotoneaster. I don't think there's any need for frantic googling in this case! It sounds very much as if the person who spoke to you had misheard someone talking about cotoneasters being susceptible to one of the known culprits that you mention, as indeed they are.
As for oaks, to take the converse, most are kindly trees that shelter and protect plants growing beneath and around them. The deciduous oaks in particular are wonderful nurses for lilies.

[Fleur Pavlidis]

But Heidi Gildemeister says we must only grow plants which don't need watering under oaks...

[Marilyn]

Phew! Thanks for that guys and gals. Re: Fleur's comment, I can vouch for the drought resistance of Cotoneaster lacteus in the garden here, we have a heap of them who had their irrigation switched off last year and didn't bat an eyelid. Not certain how they would hold up against the added root competition of a tree though.

[Fleur Pavlidis]

I planted what may be Cotoneaster lacteus round the trunk of an almond tree - I wanted it to look like a ruff and sometimes it does. It doesn't grow very enthusiastically even though it's on a drip, but that's OK, less pruning for me. I'll try taking it off irrigation in view of what Marilyn says - it'll be better for the almond I think.