Managing Million Bells

By Sarah Jandricic and Chevonne Carlow

It’s that time of year again, when baskets of Million Bells (Calibrachoa) are going up in the greenhouse.  Here’s how to deal with and prevent some of their most common issues.

Fe def calibrachoa

Iron deficiency in Calibrachoa.  The resulting yellowing can look similar to symptoms caused by black root rot or nitrogen deficiency.

From a nutritional standpoint, the best thing you can is keep the pH of your calibrachoa in its ideal range; between 5.5 and 6.0.  A pH higher than this can inhibit nutrient uptake, especially micronutrients such as iron. 


Iron deficiency can be difficult to distinguish from other issues (like Black Root Rot – see below), but typically leads to yellowing of new growth.  Leaves may only show chlorosis between the veins, or it may be spread throughout the leaf.  This is different from nitrogen deficiency where yellowing occurs in the oldest leaves. If iron deficiency occurs, adding a chelated form of iron is best for uptake.

Yellowed plant growth (yellow circle) and dead plugs (orange circle) on a plug tray of Callibrachoa.
Yellowed plant growth (yellow circle) and dead plugs (orange circle) on a plug tray of Callibrachoa from black root rot.

Million bells are also highly susceptible to Black Root Rot (Thielaviopsis) – I’ve seen this take out a good chunk of a crop.  Symptoms include:

  • Stunting of foliage and roots
  • Plants in a tray will have uneven heights
  • black areas on roots
  • yellowing of leaves

Prevention is worth a pound of cure with this disease, as it is difficult to eradicate once established.  Important steps to take include:

  • Proper Sanitation. To avoid an issue with Black Root Rot year after year, immediately dispose of  diseased plants, limit water splashing, and sanitize benches, floors and used pots/plug trays.  Always physically wash surfaces  with a cleaner to remove organic matter, then follow up with a  disinfectant such as KleenGrow (ammonium chloride compound).
  • Consider prophylactic applications of fungicides on plug trays.  Products include Senator (thiophanate-methyl) or Medallion (fludioxonil). Preventative applications are an especially good idea if you’ve issues in the past. Adding bio-fungicides containing Trichoderma harzianum (e.g. Rootshield, Trianum) may also help
  • Lowering your pH. This disease is significantly inhibited by a lower pH – between 5.0 and 5.5.
  • Manage fungus gnats and shoreflies, since these insects can spread Black Root Rot between plants. Treatments include nematodes, Hypoaspsis mites , or applications of Dimiln (diflubenzuron) or Citation (cyromazine).

If already established, rotated applications of Senator and Medallion may limit Black Root Rot, but are unlikely to cure it.

Aphids tend to be found on flowers and new growth of Calibrachoa.

Lastly, Million Bells are highly attractive to aphids.  With baskets hung up in the greenhouse, they can be “out of sight, out of mind”, but  regular monitoring is needed to prevent large aphid outbreaks.  Place sticky cards directly in baskets, and routinely check plant material for aphid cast skins and honeydew.

Once aphids are detected (and they will be!), applications of  Beleaf (flonicamid), Enstar (kinoprene) or Endeavor (pymetrozine) will usually take care of them.  However, be aware that all of these insecticides take around 4-5 days to start causing aphid death.




6 thoughts on “Managing Million Bells

    1. Hi Frank. Senator is an older, harsher fungicide, so there is some evidence that it can have negative effects on predatory mites. For example, the Biobest database indicates it is highly toxic to P. persimillis, and moderately toxic for Amblyseius speceies of mites. However, the site doesn’t indicate how the mites were exposed to the chemical (direct contact vs. residue). I’m guessing these were direct toxicity tests. So, if you’re able to apply to the soil without hitting the foliage too much, there will likely be fewer side effects. However, you may need to reintroduced your predatory mites again within 1-2 weeks of Senator application, just to be on the safe side.
      Interestingly, the soil-dwelling bios all seem fine with Senator: it has low toxicity to Hypoaspis mites, and is compatible with Atheta coriaria (I did that compatibility testing myself) and beneficial nematodes. Senator is also compatible with bio-fungicides like Rootshield. Hope this helps.

    1. Hi Gerben! Biocontrol of aphids onMillion Bells is tricky. As you know, we don’t have great bios for foxglove aphid control. And, for green peach and melon aphid, there have been reports in Ontario that the parasitoids don’t like to attack them on million bells. Micromus may be an option and I believe Rose may investigate this in the future. For now, pesticides are the best option, but I certainly hope this changes in the near future.

  1. When people are trying to cultivate plants, especially delicate ones, it can be a challenge to keep pests at bay. Thanks so much for sharing your insight on taking care of million bells.

  2. Great stuff Sarah, I can use this as reference to our case study as reference. Thank you so much for sharing a glimpse of your green house.

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