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