Annuals doing well then suddenly kaput

We often see impatiens and other annual flowers collapse in midsummer...

...once it gets good and hot. High on our list of things to look into is whether the plants doing that are the ones that were injured early in the year:

  • Their stems may have been nicked. Two possibilities: Cutworms can injure a stem as can a person who jerks too hard in depotting.
  • Their roots or crown may have been injured and opened to infection by being placed into cold soil. That can happen to some plants in a bed and not others.
  • Whole plant stress, when a plant may have had to put up with just a bit more than others around it, perhaps because it had been planted above compacted subsoil.

These plants show us their injuries only when they can no longer limp along. When the maximum strain is placed on its vascular system, such as when it needs more water than ever before on a hot windy day, a plant that's been on the edge simply shuts down.

It's important to note that any of a number of fungi may seize one of these opportunities to infect the plant, such as Botrytis, Alternaria and Phytophthora. However, those pathogens can be in the soil and yet be unable to successfully attack intact, unstressed plants.

Symptoms separate the groups

Dropping of leaves and then branches shriveling, with stem by stem progression usually indicates stems infected at their bases. That would fit stem rot or stem canker problems that can show up in midsummer although they began at planting-out time or even greenhouse-growing time as small injuries from mechanical damage or pin-point dead spots from cold soil or frost.

Where failing plants are here and there in a bed, mechanical injury is more likely -- a nick here and there at planting time.

If all the failing plants are on the outside edge of the bed, nearest the lawn, we know that set of plants may have been colder at planting time. Lawn-covered ground does not radiate as much heat as bare ground, thus on a cold spring night the plants near turf aren't so well protected from light frosts.

Some species such as Celosia and Impatiens are very susceptible to cold and related damage. However, even species such as Petunia and annual geranium (Pelargonium) which are very tolerant of cold once established are not so tolerant when small. They can still be damaged by early set-out into cold ground.


New since this article was first published:

Impatiens downy mildew is now epidemic in the Eastern U.S. Unlike the other pathogens mentioned here, it's very aggressive and infects even healthy plants, killing whole plantings rapidly and contaminating the soil for years.