mint turning purple?

kea2March 19, 2008

I've been growing spear mint in a rectangular window box, and it was doing fairly well this winter (I'm in a subtropical climate) but recently some of the leaves have started going purple and in some cases yellow. Here's some pictures.

Is this a phosphorus deficiency? Or too much fertilizer?

It also seems to be putting more energy into growing runners than into growing bigger leaves. Is there anything I can do to nudge it the other way?

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ksrogers(EasternMass Z6)

Might be too much water, as thats what causes yellowing leaves. Purple color may be just a natural state if its in strong sunlight. Judging of the small yellow leaf on the left side of the bottom photo, any watering should be done at the soil level and not applied to leaves. Phosphorous usually has no efect on color, is its mostly for roots (runners!) and blossoms. Because its in a pot, a fertilizer can be useful. Something with a slightly higher nitrogen level. Additionally, as mentioned in another recent thread, iron might be low, so use a small dose of Ironite liquid as a suppliment to fertilizer. If more leaves tend to get yellow and spotty, it could be spider mites. In the upper photo, that single leaf at the bottom may just be getting old, and will soon drop off. Sometimes seasons will prompt plants to send off runners, so that might also be normal for now.

    Bookmark   March 19, 2008 at 1:05PM
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francescod(6b/7a VA)

What you see is normal. The growth you are seeing is normal. The purple leaves are old/ last year's growth. The plant is focusing on new growth and "abandoning" the old, less vigorous growth. Notice that the new growth (on top) is all green, healthy and beautiful. Remove all the old woody stems to encourage the new growth. If your new growth becomes discolored then there may be a problem.

You probably need to repot in a larger pot. Your window box will be too small before you know it. Mint is very fast growing and quickly outgrows even large pots. I just repotted some 2 1/2 inch pots into 8 inch pots about a week ago and put them in my 60ºF greenhouse. The new transplants have already put out about 4 inches of top growth and will probably fill out the pots in 3 to 4 weeks.

Spearmint doesn't tend to discolor under strong light. Even if it did, the sun isn't strong enough at this time of the year. Peppermints tend to get darker/more purple under high light conditions, but they are supposed to. Leaves will be smaller under high light, however. Lower light conditions cause the plant to put out larger leaves in order to catch as much light for photosynthesis as possible-kind of like putting up a bigger sail to catch a light breeze on a sail boat. They also tend to be more tender and potentially more prone to disease.

It is nearly impossible to over water mint so it is not that. Water splashing on leaves does not cause yellowing in and of itself. I grow hundreds of mints per year (about 15 varieties). They, and all my other thousands of plants, all get watered from overhead (by hand, nearly every day in the late spring) and the leaves get wet. They even get fertilizer water on them. No problems with yellowing. Now, if the leaves stay wet for too long then there can be disease problems, but that is a different issue. I do water early enough in the day to allow the leaves to dry out before night fall.
Even though phosphorus is used by plants in the formation of roots and flowers, a phosphorus deficiency can show up as discolored leaves-sometimes the underside is purple, in some plants necrotic spots develop on the leaves.

    Bookmark   March 19, 2008 at 10:05PM
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I'm glad to hear that it's normal. I thought it was a fertilizer issue because I have another mint growing hydroponically in a bucket of clay pellets right next to the window box, and those leaves are a much paler (though still healthy looking) green.

I grow the mint on the northeast side of the house where it gets some sun in the morning but is shaded in the afternoon. Last year, I think the full sunlight last summer was too strong for it, and the leaves were going brown and crispy on the edges. It seems happier in partial shade. I don't typically pour water on the leaves, but it has been out in the rain a bit lately.

I'll try using a little more nitrogen fertilizer. Is it easy to over-fertilize mint?

    Bookmark   March 20, 2008 at 1:07AM
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I find that my mint goes a bit purple with the cold - a bit like my hands and feet! I don't worry about it at all. As long as the new growth is healthy, everything's just fine. I also live in the subtropics, where temperatures are mild, but perhaps you had a bit of a cold snap overnight recently?

Don't overdo the fertiliser. It's as easy to kill any plant with too much fertiliser as it is to ignore it completely. A handful of compost or fresh potting mix to top up soil levels in the pot, or a repotting in fresh potting mix, is all you really need to do. If you must fertilise, use a general-purpose, slow-release one according to manufacturer's instructions.

You can overwater mint, but it isn't an easy thing to do if it's in a pot with adequate drainage holes. A bit of subtropical rain isn't going to do it any harm at all - yep, I'm in the subtropics, too. I have only ever lost one plant due to overhead watering/rain - and that was Lamb's Ears, whose leaves absorb water.

    Bookmark   March 20, 2008 at 3:06AM
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We had a pretty bad winter, by subtropical standards. I think they said it was the coldest winter in 40 years. All the houses here are uninsulated concrete boxes with no central heating, so it wasn't much fun for the humans either.

    Bookmark   March 20, 2008 at 7:30AM
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ksrogers(EasternMass Z6)

All my outdoor thyme is purple now too.

    Bookmark   March 20, 2008 at 2:07PM
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Hi there,

I know this is really old but it's exactly what is happening to my mint and I'm pretty worried about it. Was your mint plant ok in the end or did you have to re-pot it or fertilze it?

I have found the new leaves that are in the sunlight are going a little purple as well but maybe they are just cold?

    Bookmark   April 23, 2010 at 4:27PM
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I am curious too as to how things went. I recently purchased and then repotted a mint plant. I have kept it watered and I place it in front of the window most days. I noticed I have three purple stems, but mine seemed to have started not as old growth that may be dying off, but almost like suckers that have tiny leaves, the stalks are purple, they are growing out of the pot and downward and they are tougher stalks than the rest of the plant, not the dainty stuff that makes up the rest of the plant.

    Bookmark   January 24, 2014 at 6:48PM
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