Lemongrass that shows purple or red is usually reacting to its surroundings. Most of the time the cause is cool nights, a reddish stalk base that’s normal for the plant, or stress from sun, dry soil, or hungry roots.
In most cases, lemongrass turning purple is a harmless response to cool weather or mild stress, and the plant goes back to green new growth once nights warm up and care is steady.
The color itself isn’t the danger sign. What matters is where the purple shows up, how fast it spread, and whether the stalk bases still feel firm. Once you read that pattern, you can pick the right fix and skip the guesswork with fertilizer.
The Short Answer: Check the Pattern Before Treating
Purple lemongrass is a clue, and the pattern of the color points to the cause. Before you feed, repot, or spray anything, take two minutes to look closely at the clump. If you are stocking up, a pot with drainage holes is the first thing I would pick up.
If the purple is spreading and the stalks are going soft, read my guide on why a lemongrass plant is dying. For the everyday basics, see how to care for a lemongrass plant.
Here’s how I’d read the most common patterns:
| What you see | Most likely cause | First step |
|---|---|---|
| Purple or red only at the stalk base | Normal pigment | Nothing; keep caring as usual |
| Purple tint on outer blades after cool nights | Cold stress | Watch the forecast; protect below 50°F |
| Dark green blades turning purple, slow growth | Phosphorus shortage or cold, wet roots | Check drainage and soil temperature |
| Red tips on blades in full summer sun | Sun or heat stress | Water deeply; check soil moisture |
| Orange-red raised streaks or dusty spots | Rust disease | Remove infected blades |
| Soft, mushy, dark stalk bases | Root rot or cold damage | Stop watering; inspect roots |
If the purple is smooth, sits on older outer blades, and the new center shoots are green, you’re almost always looking at stress the plant can shake off.
When Is Purple or Red Coloring Normal?
Some purple and red on Cymbopogon citratus, the West Indian lemongrass sold for cooking, is perfectly normal. The plant makes pigments called anthocyanins, the same ones that color red cabbage and fall leaves, and they show up most at the base and during cool spells.
Reddish Stalk Bases vs. Discolored Blades
A purple, reddish, or brownish band at the bottom of the stalks is part of how many lemongrass plants look.
A purple to brown color around the stalk base can be normal, while the same color spreading up through the foliage deserves a closer look.
So start at the soil line. If the bases are firm and tinted but the blades above are green, you have a normal plant.
If the blades themselves are turning purple or red, keep reading. That’s where stress, nutrients, or disease come into play.
How Cool Weather Brings Out Purple Pigments
Cool nights slow down how fast lemongrass can move sugars out of its leaves. Those sugars build up, and the plant turns some of them into purple pigment.
You’ll see this most in late September and October across the Northeast, Midwest, and Pacific Northwest, and in spring right after planting out. It’s the same response gardeners notice on many warm-season plants as fall arrives.
The tint shows up first on outer, older blades. Healthy inner shoots stay greener, which is a good sign the plant is fine underneath.
How Do Cold Nights Affect Lemongrass?
Cold is the most common reason lemongrass turns purple, because this tropical grass starts struggling once nights drop below about 50°F.
It’s hardy outdoors only in USDA zones 10 to 11, and in zone 9 with protection, so most US gardeners deal with chill every fall.
Signs of Cold Stress Below About 50°F
Growth slows sharply below 50°F, and you’ll notice changes within a week or two of chilly nights:
- Purple or reddish tint on outer blades
- Pale, dull blades and slower new growth
- Brown tips once nights get near 40°F
- Limp, water-soaked blades that turn brown or black after a frost
The first two are mild stress. The water-soaked, collapsing look means real cold damage, because freezing ruptures the plant’s cells.
Repeated nights in the 35°F to 45°F range can cause a slow decline that looks like underwatering. If the soil is moist and the weather has been cool, blame the cold first.
Protecting Outdoor Clumps and Moving Pots Indoors
My advice is to bring potted lemongrass indoors before nights fall below 50°F. The Royal Horticultural Society sets 45°F as the move-in point, but I’d go a little earlier so the plant never gets stressed.
For in-ground clumps:
- In zone 9, cut stalks back to 6 to 12 inches and cover with frost cloth on cold nights, weighting the edges with bricks.
- Add 3 to 4 inches of mulch over the root zone.
- In zones 8 and colder, dig up a few sections with roots and pot them before the first frost.
Indoors, give pots the sunniest south-facing window or a grow light for 12 to 14 hours a day. Water sparingly over winter. If you want more detail, my guide to grow lemongrass indoors covers winter setup.
Is Phosphorus Deficiency the Cause?
Phosphorus shortage can turn lemongrass purple, though in home gardens it’s less common than cold. The two are closely linked, so it helps to check soil conditions before buying a high-phosphorus fertilizer.
Purple Leaves and Slowed New Growth
When a plant can’t get enough phosphorus, sugars pile up in the leaves. Blades first turn a dull, dark green, then pick up a purple tint, often starting on older leaves.
Other clues point the same way:
- New shoots come in thin and small
- The clump stops spreading new stalks
- Growth stalls even in warm weather
If your plant shows purple blades along with stalled growth in midsummer heat, a nutrient problem moves higher on the list.
Why Cold, Wet Roots Can Mimic a Deficiency
Cold, soggy soil blocks phosphorus uptake even when plenty of it is in the ground. That’s why spring transplants set out too early often turn purple and then green up on their own once the soil warms.
Wait to plant outdoors until nights stay above 60°F. Use well-draining soil or a raised bed so roots aren’t sitting in water.
If the weather is cool and the soil is wet, fix those two things and give the plant two to three weeks before adding phosphorus. Extra fertilizer won’t help roots that are too cold to use it.
Can Strong Sun and Heat Cause Red Tints?
Intense sun and heat can bring out red or reddish-brown tints, especially on blade tips. Lemongrass loves full sun and handles heat well, so this is usually a sign of another stress, like dry soil, piling on top of the sun.
Distinguishing Sun Stress from Normal Pigment
Normal pigment sits at the stalk base and stays smooth. Sun stress shows on the parts of the blades facing the afternoon sun, often at the tips.
Look for these sun-stress signs:
- Red or bronze tips on the sunniest side of the clump
- Papery, crispy brown edges on the worst blades
- Color appearing after a heat wave, often above 95°F to 100°F
New plants moved straight from a shady nursery into full sun show this most. Hardening them off over a week helps.
Keeping Full-Sun Plants Healthy During Heat Waves
Keep lemongrass in full sun, since it needs 6 or more hours of direct light to grow strong stalks. Read how much sun lemongrass needs if you’re deciding on a spot.
During heat waves in the Southwest, Texas, and the Southeast:
- Water deeply in the early morning.
- Add 2 to 3 inches of mulch to keep roots cool.
- Move pots off hot concrete or dark decking.
- Use 30% shade cloth for afternoons only if tips keep scorching.
Most red sun tints stay on the old blades. New growth comes in green once the heat breaks.
What Does Drought-Stressed Lemongrass Look Like?
Dry lemongrass shows red or brown tips, curled or folded blades, and outer leaves that dry out first. The whole clump looks dull and droopy, and the pot feels light when you lift it.
Drought and sun stress often show up together. Lemongrass is a thirsty plant that likes evenly moist, well-drained soil, so a few dry days in summer can be enough.
Checking Soil Moisture Before You Water
Push your finger into the soil up to the second knuckle, about 2 inches. If it’s dry there, water deeply until it drains out the bottom of the pot or soaks 6 inches down in the ground.
If it’s still damp, wait. Overwatering a plant that’s purple from cold makes things worse, because cold, wet roots rot fast.
Why Containers Dry Out Faster in Summer
Pots heat up on all sides, and the small amount of soil runs dry quickly. In July a container can need water every day, especially terra-cotta in full sun.
Fabric grow bags breathe well and drain fast, which roots love, but they dry out even faster than plastic. Pick a pot at least 12 to 18 inches wide for a mature clump. My guide to grow lemongrass in pots covers sizes and soil mixes.
Is It Rust, Leaf Spot, or Pest Damage Instead?
Disease and pests cause spotted, streaky, or speckled color, while stress causes a smooth tint. That difference is the fastest way to tell them apart.
Orange-Red Rust Streaks vs. a Smooth Purple Tint
Rust is a fungal disease that forms small raised bumps, called pustules, on the blades. They’re orange to reddish-brown and leave a dusty powder on your finger when you rub them.
| Feature | Rust | Stress tint |
|---|---|---|
| Texture | Raised, powdery bumps | Smooth, flat color |
| Pattern | Streaks or dots along blades | Even wash of color |
| Rubs off | Yes, orange dust | No |
| Weather | Warm, humid, wet leaves | Cool nights, dry spells |
Rust spreads in warm, humid weather, so it’s common in the Southeast and Gulf Coast. Overhead watering and crowded clumps