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help! my young cosmos plants are under attack.

Hello I raised some cosmos from seeds in February and now they are in flower! However, they are not well at all. Their poor leaves are turning yellow. I can see that some tiny bugs are all over the under side of the leaves and there are spider Web like structures here and there on the plants. What can I do to save them? Can I get some chemical spray or are they too young for chemical?


  • Have you tried squashing them? That's what I tend to do when I spot anything undesirable on the underside of leaves.

  • BobTheGardenerBobTheGardener Posts: 11,384

    Hi flowerlover, that is almost certainly red spider mite.  I take it you are growing them indoors or in a greenhouse etc?  Some control can be had by spraying the plants regularly with water as the mite doesn't like high humidity, but it won't kill them.  Most bug sprays are useless as the mite has evolved immunity to most pesticides, so the only guaranteed way to totally eradicate them is biological warfare using a predatory mite called Phytoseiulus.  The predators can be bought online for about £12 for 1000.

    A trowel in the hand is worth a thousand lost under a bush.
  • Thank you, NGCARDS. They are very tiny and hard to aim at, I am scared that I might do some more damage by squashing the weak leaves too hard.

  • LynLyn Posts: 23,190

    To be honest Flower, i would bin those, cosmos grow so quickly this time of year, they will be up and away in a week, and so much stronger,  you can then plant out on May.  if yours are flowering now, they will be over before summer had started.

    I sowed some last year in June, on Monty Dons advise and they flowered through till oct/ nov time. Its not worth messing about with infested seedlings.

    Dont let them flower until you have a big plant, pick any buds off as they will sap the strength of the plant until its bigger.

    Gardening on the wild, windy west side of Dartmoor. 

  • Hi Bob. Thank you for your advise. The cosmos are on a sunny windowsill above a radiator. I will try to get hold of a possibly smaller packet of these friendly insects. I think 1000 is a bit too many. I have only 5 plants, wouldn't want them little guys to starve.. 

  • Thank you, Lyn. Oh, people are so helpful here. I will sow some more. and some nipping to been done too when I get home. image

  • LynLyn Posts: 23,190

    Bin them flower, its not worth buying sruff for those seedlings, i have learned over the years that ruthless is the best policy, feb sowing was far to early for cosmos. They will be short lived, just start again and put it down to experience.

    As soon as the seeds are about 2inches, get then out somewhere cooler, plastc green house or cold frame, indoors is too warm for them.

    Gardening on the wild, windy west side of Dartmoor. 

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