Rosemary question - Ask Extension
something is not right with my rosemary... the leaves are spotted... is there a book about herb issues & solutions that you know of?
Knowledgebase
Rosemary question #818127
Asked January 05, 2023, 1:06 PM EST
something is not right with my rosemary... the leaves are spotted... is there a book about herb issues & solutions that you know of?
Cecil County Maryland
Expert Response
To best help you we'd like to know more.
Where and how are you growing your rosemary? Indoors/Outdoors? When was it planted?
Do you know what variety it is?
If you could answer the questions and attach a couple of photos showing the spots to this reply, we'd be happy to take a look.
In the meantime, here is our Rosemary page:
https://extension.umd.edu/resource/rosemary
Christine
Where and how are you growing your rosemary? Indoors/Outdoors? When was it planted?
Do you know what variety it is?
If you could answer the questions and attach a couple of photos showing the spots to this reply, we'd be happy to take a look.
In the meantime, here is our Rosemary page:
https://extension.umd.edu/resource/rosemary
Christine
Good Morning,
One is growing on the south side of my shop. It was planted 2 years ago. That is the one that is real bad.
The 2nd is in a large pot in my greenhouse & that one is just starting to show the spots. Planted 3 years ago.
kind of a tan color? I have attached 3 pics.
Thanks,
Liz
One is growing on the south side of my shop. It was planted 2 years ago. That is the one that is real bad.
The 2nd is in a large pot in my greenhouse & that one is just starting to show the spots. Planted 3 years ago.
kind of a tan color? I have attached 3 pics.
Thanks,
Liz
Hello Liz,
Thank you for the photos. The rosemary that is completely brown is probably dead, though you can cut it back if you prefer to see if new growth emerges from the roots in spring. Rosemary are marginally winter-hardy here and it's not uncommon for them to succumb to our winter temperatures when grown outdoors (either in-ground, which is more insulating, or in a pot). Another key factor in survival is soil moisture and drainage; they are very intolerant of soggy conditions or soils that don't drain well or which are over-watered. If the temperatures alone didn't cause damage, perhaps the roots stayed too wet at some point. In that case, regrowth will not occur and the plant would need removal. Raised beds can help improve drainage greatly in areas with conditions that might not suit the plant if it were grown directly in the ground. While potted plants tend to have much better drainage because potting soil is usually more free-draining than in-ground soil, sometimes the exposure of the root ball to the cold causes damage instead.
The leaf speckling is caused by either a sap-sucking insect or by mites. Both are fairly common for rosemary and a variety of other herbs. Thrips and leafhoppers are among the most common of the insects that can cause this type of feeding damage, and they can be hard to tell apart from symptoms alone. None of these pests are active now; the damage was done last season and will not heal since plants cannot reverse leaf damage. However, the leaves can still photosynthesize and feed the plant, so don't necessarily need removal. If you haven't used an insecticide on them, they will also be safe to cook with; as always, just give them a good rinsing. If you have used an insecticide like horticultural oil (neem oil being one type) or insecticidal soap, be sure to check the product label about how long to wait before harvesting. Either treatment can work to suppress populations of thrips, leafhoppers, and mites, but must be applied only when the pests are present and actively feeding, so at this point will have to wait until spring or summer.
A simple spray of water can help knock-down the numbers of mites or insects in the meantime, and may suffice without insecticide use if done regularly. (Although it can be done any time of year, avoid too much wetting of the foliage in winter since airflow in the greenhouse will be minimal and wet leaves that don't dry quickly are more vulnerable to infection.) When using water to remove pests, spray the foliage with a strong jet, like from a garden hose, and be sure to hit the upper and lower surfaces of the leaves. Tipping the pot on its side can help with this, though try to not blast soil out of the pot in the process, which can be challenging. Plants experiencing stress from drought or under-watering while in pots could be more vulnerable to damage from these types of pests. Even though rosemary appreciates drying out a bit between waterings, make sure it's getting monitored regularly for watering needs by feeling the soil and watering when it's becoming fairly dry to the touch an inch or so deep into the soil. If using a saucer under the pot, empty it promptly after watering so the drained water doesn't seep back into the soil and drown roots.
If the plant's new growth next year looks normal and stays unspotted, the problem has probably resolved itself, though you can continue to monitor the young leaves for signs of damage and treat (either with water or the aforementioned low-toxicity pesticides) as needed. Be sure that any pesticide you choose is labeled for both the plant type (herbs or edible plants in this case) and the pest in question, and always follow application instructions regarding dosage and timing. Don't substitute a home remedy as this either won't work as well or risks damaging the plant further in the process. Be aware that a horticultural oil spray, even when applied as directed, might cause a bit more damage to a plant already heavily-stippled by pest feeding, but that will be unavoidable; you can try cutting back heavily-damaged foliage as an alternative.
Miri
Thank you for the photos. The rosemary that is completely brown is probably dead, though you can cut it back if you prefer to see if new growth emerges from the roots in spring. Rosemary are marginally winter-hardy here and it's not uncommon for them to succumb to our winter temperatures when grown outdoors (either in-ground, which is more insulating, or in a pot). Another key factor in survival is soil moisture and drainage; they are very intolerant of soggy conditions or soils that don't drain well or which are over-watered. If the temperatures alone didn't cause damage, perhaps the roots stayed too wet at some point. In that case, regrowth will not occur and the plant would need removal. Raised beds can help improve drainage greatly in areas with conditions that might not suit the plant if it were grown directly in the ground. While potted plants tend to have much better drainage because potting soil is usually more free-draining than in-ground soil, sometimes the exposure of the root ball to the cold causes damage instead.
The leaf speckling is caused by either a sap-sucking insect or by mites. Both are fairly common for rosemary and a variety of other herbs. Thrips and leafhoppers are among the most common of the insects that can cause this type of feeding damage, and they can be hard to tell apart from symptoms alone. None of these pests are active now; the damage was done last season and will not heal since plants cannot reverse leaf damage. However, the leaves can still photosynthesize and feed the plant, so don't necessarily need removal. If you haven't used an insecticide on them, they will also be safe to cook with; as always, just give them a good rinsing. If you have used an insecticide like horticultural oil (neem oil being one type) or insecticidal soap, be sure to check the product label about how long to wait before harvesting. Either treatment can work to suppress populations of thrips, leafhoppers, and mites, but must be applied only when the pests are present and actively feeding, so at this point will have to wait until spring or summer.
A simple spray of water can help knock-down the numbers of mites or insects in the meantime, and may suffice without insecticide use if done regularly. (Although it can be done any time of year, avoid too much wetting of the foliage in winter since airflow in the greenhouse will be minimal and wet leaves that don't dry quickly are more vulnerable to infection.) When using water to remove pests, spray the foliage with a strong jet, like from a garden hose, and be sure to hit the upper and lower surfaces of the leaves. Tipping the pot on its side can help with this, though try to not blast soil out of the pot in the process, which can be challenging. Plants experiencing stress from drought or under-watering while in pots could be more vulnerable to damage from these types of pests. Even though rosemary appreciates drying out a bit between waterings, make sure it's getting monitored regularly for watering needs by feeling the soil and watering when it's becoming fairly dry to the touch an inch or so deep into the soil. If using a saucer under the pot, empty it promptly after watering so the drained water doesn't seep back into the soil and drown roots.
If the plant's new growth next year looks normal and stays unspotted, the problem has probably resolved itself, though you can continue to monitor the young leaves for signs of damage and treat (either with water or the aforementioned low-toxicity pesticides) as needed. Be sure that any pesticide you choose is labeled for both the plant type (herbs or edible plants in this case) and the pest in question, and always follow application instructions regarding dosage and timing. Don't substitute a home remedy as this either won't work as well or risks damaging the plant further in the process. Be aware that a horticultural oil spray, even when applied as directed, might cause a bit more damage to a plant already heavily-stippled by pest feeding, but that will be unavoidable; you can try cutting back heavily-damaged foliage as an alternative.
Miri
