2 Reasons Why Your Rosemary Leaves Are Turning Red/Purple

It can be alarming to see the foliage on your rosemary, turning in different shades of red and purple. It can be difficult to pinpoint the exact cause, but rosemary foliage turning red or purple is a sign that something is wrong, and you need to check out.

Red-purple foliage and sometimes a stronger scent on rosemary plants are usually caused by plant distress or nutrient deficiency. In the first case, temperature, water issues, or disease are very often the culprits while phosphorus deficiency in the second.

Being able to pinpoint the exact cause of the problem is fundamental to undertake the right action.

Rosemary Stress (and the 3 Cause)

The red/purple discoloration is often a sign of stress in the majority of herbs and houseplants. The coloration is due to the build-up of a red or purplish pigment called anthocyanin that builds up when a plant is stressed, and normal plant processes are interrupted.

What are the causes of plant distress that cause such red/purple colors? Temperature, water, and diseases.

Temperature – The discoloration may have occurred because your rosemary plant was exposed to temperature extremes (did you place your rosemary close to a radiator, left it outside during a frost). According to studies, anthocyanin has a role in protecting plants against extremes of temperature that, if you want to avoid any problem, should be kept between 60 and 70F (16 to 21C).

Water – Chatting with other gardeners, more often than not, overwatering is an issue. Watering too much causes the rosemary root to start rotting. Among other signals (like wilting), you might also see the leaves getting purple.

Disease – Environmental problems may also have contributed to conditions where the disease was able to take hold. If your rosemary plant is also showing other signs of ill-health (wilting, yellow leaves, white dust forming on the leaves), it may have a fungal disease or another disease problem.

Often these are triggered by a high humidity environment and/or soil that has not been treated before (like garden soil). This can also cause abnormal purple pigmentation through the disruption of normal plant processes.

Phosphorus Problems

When it comes to your rosemary, this is relevant because red/purple pigmentation can be a sign that the plant is not getting the phosphorus it needs. This is the same phenomenon that explains why fall leaves get their red colors in late summer: a reduced level of phosphate in the leaf, as discussed in this study.

Here’s the catch

Rarely, a phosphorus deficiency in your growing medium is to blame for red/purple discoloration. Indeed, more often than not, waterlogging and other environmental issues are the causes. Rosemary is a tough and hardy plant, and as a woody shrub, it will rarely require fertilization, as discussed by the University of California. In fact, overfertilization can cause more harm than good. There are only 2 cases where phosphorus deficiency can really be the problem.

The first case is when you are using a soilless growing medium (water only, for instance, check the list of growing medium). In this case, the lack of fertilizer (and then potassium) is very likely to be the issue.

The second exception can be due to a problem with pH. A very alkaline environment can make phosphorus (and iron and manganese) become less available. Here a list of the effect of pH on herbs and plants.

Fixing Stress Problems

If your rosemary only has a few affected patches of red/purple color, it might not be too late to save your plant. Read on to pinpoint the cause of the problem, and to find out how to bring your plant back from the brink.

Snip off any affected areas of the plants (no more than two-third) and then try to understand the origin of the problems using the guidance below

Water Stress

In my experience, the number one reason rosemary turns red/purple is that it has wet feet. If your rosemary develops discolored foliage, problems with watering are the first place to look. Rosemary is a Mediterranean herb, which likes moist yet very free-draining soil. It will most definitely not tolerate waterlogged roots.

Indeed, many people who grow herbs indoors make the same mistake over and over. They may have seen brown discoloration caused by a lack of humidity and watered more. This is a mistake. Keep plants in a cool, sunny spot. And if humidity is low, place your pot into a saucer of water filled with pebbles to raise humidity and reduce stress on the plants.

Here the tip

Rosemary likes taking moisture from the air, not the soil. To keep the air moist, keep the saucer filled at all times, and mist foliage if the air is very dry indoors. In general, giving rosemary a good soak every couple of weeks is sufficient. Air humidity is more important.

🌱 Growing Rosemary Indoors - How to Not Kill Your Rosemary

What to check?

The soil of your rosemary should be dry or slightly moist at the touch a few days after watering. Stick your finger till two knuckles. If the soil feels waterlogged (after 1-2 days of watering), then you found your problem.

If so, let the soil dry.

This does not guarantee to fix the problems, though, as the rosemary might be already developing root rot. In this case, after a few weeks of no watering, if your plant does not recover, just repot. Extract the plant, cut all the roots to look brown and slimy, let them dry, and plant in a clean (from the shop, as free of nasty bacteria) growing medium.

A typical cactus potting mix (as this one on Amazon) is a good start. Remember to add some compost (a third of the volume, here a good one). If you do not want (or do not have), you can opt for a good quality potting mix (here my favorite) and add some perlite (here the one I usually buy, a good one on Amazon).

Temperature Stress

Have you changed the position of your rosemary? If so, this might be the culprit.

Perhaps you moved your rosemary close to a very sunny window as summer arrived. In this case, you need to remember that the glass can get quite hot, and you do not want your rosemary to touch it.

Did you move it outdoors as the spring arrives? No problems, but rosemary is an alive organism. You cannot move rosemary outdoors all of a sudden. You need to let it gradually adapt to the outdoor temperature. How?

Start moving your rosemary 2-3 hours a day only (avoiding the hottest hours) outdoors during spring. Move it indoors for the rest of the day. This will allow the plant to slowly adapt to the outside temperature. Increase 1-2 hours every other day until (after 1 week or so) you can leave, at all times (in warm seasons) your rosemary outdoors.

Disease and Pests

As discussed also by the authoritative RHS, rosemary is, differently from “softer” herbs (like basil and cilantro), less affected by diseases or pests. However, when these happen, you should be able to spot and identify them quite rapidly and address them accordingly.

For instance, scale insects, spittlebugs, and beetles are the most common pests affecting rosemary. In all the cases their presence is quite evident (from massive beetles chewing your leaves to spider residue left by the spittlebugs).

Among diseases, powdery mildew is quite common on rosemary (and many other herbs). You can easily spot such fungi-related diseases by the white coating that you will find on leaves and stems.

Hence, if your rosemary is affected by one of the above, before worrying about purple leaves you should be worried about how to get rid of such problems in the first place.

★ How to: Treat Powdery White Mildew (A Complete Step by Step Guide)

Phosphorus Deficiency in Rosemary

As mentioned above, although rarely, the problem might lie on a phosphorus deficiency. To be 100% sure, what I would do is to check the soil phosphorus level. How?

Do not be scared; you do not need an expensive lab test (considering that you might have small potted rosemary). You can use the inexpensive Luster leaf rapitest (here on Amazon). If you want to know more on those inexpensive tests, check this detailed review and step by step guide on potting soil testing.

Depending on the results of the test, you have two options:

1. In the case of phosphorus deficiencies, you can feed the rosemary with a liquid feed fertilizer. I suggest either an organic version (as way milder than chemical ones) as the Dr. Earth one (should be quite cheap, here its current price on Amazon);
   
2. In case of phosphorus excess (or if solution 1 sounds too much of a hassle), you can replace the soil. As mentioned before, the same type of cacti soil (or potting mix and perlite) can be used.

DIY Hack

In case the phosphorus deficiencies are correlated with alkaline conditions (again, the Luster leaf helps on that) are the problem, adding apple cider vinegar or other mildly acidic substances to your liquid feeds may help bring pH down to more acceptable levels.

Your Takeaways

1. Red-purple rosemary leaves are usually a sign of stress.
2. Stress can be caused by temperature or watering problems (or disease).
3. Most often, waterlogging is the problem – rosemary hates having wet feet.
4. Choosing the right container and growing medium, and watering right usually keeps rosemary happy.
5. Occasionally, phosphorus deficiency is to blame. In this case, the quickest solution is the right potting mix.

Further Questions

Can you revive dead rosemary? Rosemary, as well as many other herbs, can be grown only if its roots are alive. The only exception is from cuttings, where roots can develop after, but only if the stem is in healthy conditions.

Should you remove rosemary flowers? Differently from annuals like basil and cilantro, flowers will not lead rosemary to death, although its leaves might lose part of their fragrant taste due to a change in the amount and type of essential oils.

Are rosemary and lavender the same? Despite they look pretty similar they are not the same. Their way to growth, taste and aspect are quite different. Curios? Check the article below

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