ET Thorny problems: blanket weed (Filed: 30/06/2001)

Gardener and lecturer Helen Yemm, weeds out the answer to a tough problem

Blanket response

We have the "dreaded blanket weed" to which you referred in your reply to a letter about green pond water. We are vainly trying to deal with it by hauling it out on sticks. Any better suggestions gratefully received. Libby Rolls, Stroud, Gloucestershire

You are not the only reader to notice that I ducked the blanket weed issue. The fact is that I am fighting a battle with it myself, and I do not have much cheerful advice to give.

Blanket weed - those long strings of green "hair" that smother other pond weeds and rise to the surface in a gooey, bubbly mass on hot days - is what my local pond expert, Nicky Phipps, describes as "an opportunistic algal growth that feeds on the nitrates and general muck in pond water". Algae are spread by airborne spores, but you can also infect a pond by introducing weed from infected sources.

Eradication is a nightmare. There are effective chemical cures, such as Clarosan, but these also destroy pond weed and lilies. Numerous miracle treatments have cost me a fortune and got me nowhere. Others work a little, but create an unattractive sludge.

I am trying small, immersed pads of barley straw (as often used by water authorities), but so far these, too, are having little impact. As they rot down, they release hydrogen peroxide or some such into the water, which slows down the blanket weed. I have doubled the recommended amount and am waiting for the fish to go blond. Nothing ventured, nothing gained.

I had an interesting email from George, who has had success using magnets. Someone else told me that these only work in Northampton. Magnets? Northampton? More information - and suggestions - please, readers.

Rhodo drive

Judith Woodfield writes from Sutton Coldfield and P. Knapp emails with similar problems. Both have beautiful rhododendrons that have become straggly and overgrown, and they are unsure how and when to prune without losing next year's flowers.

It is time to be brave and rather ruthless - and to develop those qualities essential to all gardeners, patience and pragmatism.

Mature rhododendrons respond well to occasional hard pruning, but you are bound to lose some of next year's flowers. Major limbs that are in the way can be removed (in these cases, it seems they must be removed) to restore order and recover your paths.

It is fine to do it now, or immediately after they have finished flowering, treating them as you would any other spring-flowering shrub. A small, narrow-bladed pruning saw will make it easier to get between tightly packed branches.

Trust me on this one. I have done it, and it works. But be sure to be really radical, because you do not want to have to do it again for several years.

Follow up with a feed of a special fertiliser for azaleas and rhododendrons (such as the one made by Vitax), and/or a top-dressing of leaf mould or organic matter suitable for acid-loving plants (not horse manure).

New shoots will appear from seemingly budless branches later in the summer. These will not flower next spring, but may form flower buds next summer, for spring 2003. If there are several bushes that are overgrown, do one each year, perhaps, to lessen the shock to the system (yours, that is).

Old growbags

What should I do with the contents of old growbags at the end of the season? If I tip them out on my vegetable patch, will they adversely affect the pH of the soil? John Noble, Hemel Hempstead, Herts

At the end of the growing season, what is left in a growbag will be organic matter leached of most of its nutrients. This can be beneficial as a soil conditioner or moisture-retaining mulch around border plants or young shrubs, or to help in the battle against clay soil.

As far as acidity or alkalinity (soil pH) is concerned, the contents of the bags may be slightly alkaline - if the crops in them were watered with tap water containing a lot of lime. I cannot imagine, however, that this would alter the pH of your soil enough to have an impact.

After the deluge

Several of my mature forsythias died soon after flowering, whereas a younger plant that had layered itself nearby is flourishing. Could this have something to do with the wet winter we have just had? Nick Hawkins, Woking, Surrey

Most probably, yes. The mature, deep-rooted forsythias may have been sitting in water for up to six months, which they would hate. The smaller, more shallow-rooted "baby" (which probably propagated itself by forming roots on the tip of a "parent" branch that made contact with the ground - a method known as shoot-tip layering) probably dried out more quickly.

I think as the summer progresses we will see more and more shrubs complaining about last winter's waterlogging. My Elaeagnus x ebbingei is dropping far more of its old leaves than is normal for this time of year. Fortunately, the new growth is prolific, so the plant is clearly going to be fine. But it has suffered, and is not a pretty sight.

Ants revisited

There was much sympathy for Diana Richmond, who was plagued by ants in her pots. (Thorny Problems, April 28). Pauline Hadlow from Camberley has for the past two years planted her containers with penny royal (Mentha pulegium), a traditional ant repellant, along with their normal contents. So far they have been ant-free. Seeds of penny royal can be obtained through the Organic Gardening Catalogue (01932 248541).

-- Anonymous, July 06, 2001