It was a grey and windy day, spitting with rain, so I retreated to the greenhouse to prick out seedlings. The germination rate was pretty good, and I didn't have too much of a problem with mould, but I haven't been impressed by the growth rate in most cases. I don't know whether they have got too warm, despite my reasonably timely application of shading paint, or whether the seed compost was so low in nutrients as to be unable to support growth, or whether some other factor is at play.
I found one culprit on two pots, in the form of root aphid. These are tiny aphids that live, as the name suggests, on the roots of plants, where they suck sap and spread virus diseases just like regular aphids. When you tip the contents of the pot out, you will see white, fluffy growths on the roots, and possibly sticking to the inside of the pot. I had never even heard of them until about nine or ten years ago, when I first encountered them at the plant centre.
I looked at the RHS website to see what was to be done about them, but it didn't mention root aphids at all. I might email them after the bank holiday asking for advice, since I am a member and providing advice to members is one of the services offered, if only to point me to some relevant page on their site which I have failed to find. In the meantime I kept searching the vast resources of the world wide web.
Given that so many insecticides have been withdrawn, or made unavailable to amateur users, it's essential to get up to date advice. A fairly recent article by Which concluded that there was no chemical treatment, and didn't say anything about biological ones. I had a feeling it wasn't being as helpful as it might be. Many of the articles I turned up were American, so I had no idea whether any of the products mentioned were available in the UK. As well as conventional chemical insecticides, they mentioned Neem oil and lemongrass oil as being good aphid treatments. I know that Neem twigs were used as toothpicks in places like India where it grows, partly because of their natural antiseptic properties, but have never noticed Neem (or lemongrass) oil for sale. It seems that root aphids are an increasing problem for growers under glass, described as the red spider mite of the new millennium, and that as well as being a nuisance for salad growers, they are a problem for folks raising cannabis plants. I am not too proud to take advice from a cannabis grower, if I thought they knew what they were talking about when it came to root aphids.
The symptoms you will see above ground are poor foliage growth, discoloured leaves, and wilting, all signs that the root system is not working properly. I turned out the first of a tray of Geranium maderense I had planned to repot because they weren't growing too well and I thought they might need more space, but found that they had root aphid as well. I considered for a while whether I could wash the compost, and the aphids, off their roots and repot them in fresh compost, but decided to leave it until I'd had a chance to go to a decent garden centre and see if any drench were available. Apparently pyrethrum is effective agains root aphids (I think it is pretty lethal to most insects) but I would need a way of getting it down into the root system.
I almost never use insecticides outside, since I would rather leave the beneficial insects unharmed and let nature find its balance, and the birds do a very good job of keeping the plants clean, but I can't see that using pyrethrum on compost under glass is going to hurt. It is based on a chemical defence found naturally in plants, and doesn't hang around in the soil for years like some of the artificial pesticides that had such a devastating effect on the environment. Anyway, I'll have to go and see what's available, and what the RHS advises.
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