5 Reasons for Purple Stems on Tomato Seedlings

Do your tomato seedlings have purple stems? It’s not a death sentence, but you should know why!

I walked into my shed – a makeshift greenhouse – and realized my tomato seedlings had purple stems and purple leaves. They were fine the day before, so what causes seedlings to turn purple?

Talk about disappointing.

You spend all of this time taking care of your tomato seedlings, and then the next moment, it seems like they take a dive for the worse. Seedlings are finicky plants, and sometimes, the simplest disruptions cause them to change.

When I saw the purple stems, I immediately spent time trying to figure out what causes this to happen, and I learned a few things.

Purple stems on tomato seedlings may be caused by cold temperatures, a nutrient deficiency, or overall stress.

In my case, the tomato seedlings – planted in the same mix – kept inside of my house looked incredible, but the ones I put outside turned purple. The biggest difference was the temperature; our area experienced a few cold snaps, and while I kept a heat lamp in the greenhouse, it wasn’t enough.

Related: Did You Find Tomato Leaves Turning Yellow? Here are 10 Reasons!

Tomato Seedlings Present Problems in Different Ways

Tomato plants are fickle plants, and if they aren’t happy, they’re going to tell you. Seriously, I’ve had plenty of issues with these little feelings.

Your tomato plants turn purple for various reasons, but something you should know is that they won’t always look the same or be affected by the problems in the same way. Here are some of the ways your seedlings or adult plants will tell you there is a problem.

Purple Stems Before True Leaves

If your tomato seedlings are young without any true leaves, they might have purple stems. This is rather normal and won’t cause any problems later, so don’t worry.

Purple Bottom Leaves on Older Tomato Seedlings

You might notice that the bottom leaves turn purple first; this is often a sign of a phosphorus deficiency. This can be fixed!

Purple Veins on the Leaves

Take a look at the veins on the back of the seedling leaves. The tops of the leaves may look normal and green but the backs could be a dark purple. Typically, this is an indication of nutrient issues, and seedlings over two weeks old should be fertilized.

Rolled Up Purple Leaves

Not only do adult tomato plants experience leaf curl, but tomato seedlings can as well. Typically, this indicates your seedling is under stress such as overwatering, cold temperatures, light issues, or more.

Related: 8 Reasons for Tomato Leaves Curling Up in Your Garden

5 Reasons Your Tomato Seedlings Have Purple Stems

If you notice your tomato seedlings have purple stems, the first thing to do is not panic – seriously. When I saw my plants, I knew there was a reason, and that I could fix it. This problem rarely is so serious that your plants die, and once the seedlings go outside, the issue is often resolved.

What you need to do is eliminate each reason for purple leaves and stems and figure out which one it could be.

1. Cold Temperatures

I had tomato seedlings inside my house and in our shed – unheated – and only the ones in the shed turned purple. That indicated to me that something in the shed was causing stress – the cold temperatures.

Cold temperatures stunt the growth of your tomato seedlings; they cannot handle temperatures below 50F. Cold soil also reduces how much phosphorous your plants can access, hitting your poor plants with a one-two punch.

Controlling the temperatures of your seedlings is important.

You can use gardening heat mats to keep your plants toasty, or put a heater in your shed or greenhouse. I put a heat lamp in my shed when the temperatures were too low.

If possible, keep your tomato plants between 65F and 80F; this is the plants’ sweet spot. Also, if your plants are cold, avoid overwatering because it causes even more shock.

I solved my problem by bringing my tomato seedlings inside and simply keeping cold weather crops outside until the outside temperatures heat up.

2. Phosphorous Deficiency

Take a close look at your seedlings. The best way to identify a phosphorus deficiency is that the stems and bottom leaves are purple.

This problem is easy to fix, and it’s common because people forget to feed seedlings. However, seedlings need to be fertilized once they have their real leaves, so long as you dilute the strength of the fertilizer.

If you suspect that a phosphorous deficiency is the problem, here are some suggestions.

• Fish fertilizer and kelp fertilizer are two excellent organic sources of phosphorous.
• Always dilute the fertilizer at half strength until they plants are 8 to 10 inches tall.
• Consider using a general fertilizer with a 5-5-5 NPK ratio, or look for one with higher phosphorous and lower nitrogen, such as a 3-5-3.

3. Magnesium Deficiency

Another reasons for purple stems on tomato seedlings is a magnesium deficiency. While magnesium isn’t part of the NPK trio, it is crucial for chlorophyll production. If chlorophyll is lacking, the leaves turn purple or brown, so you want to make sure the plants receive magnesium.

The easiest way to do this is a foliar feed, but you also can use epsom salt. Mix half a spoon of garden epsom salt with a gallon of water and fill a spray bottle. Use this as a DIY foliar feed.

Another option is to use this foliar feed with your phosphorus fertilizer above every two weeks, killing two birds with one stone.

4. Grow Lights are Too Strong

Sometimes, the grow lights you select are too strong for your seedlings, especially if you use the new LED white lights. Studies suggest that intense white and blue lights cause plants to produce more anthocyanins for protection, leaving their stems purple as an indication of stress.

This is easy to fix!

Try moving the lights further away from your plants to reduce the intensity. You also could try other grow lights; some are dimmable!

5. Other Stress Factors

Sometimes, other stress factors may cause your tomato seedlings to have purple stems. For example, planting them in close quarters could cause stress because their roots have fewer places to grow and spread out.

This happens when you have multiple seedlings in one cell, and you have yet to up-pot them. Sometimes, the answer is as simple was transplanting them into bigger containers.

Related: How to Prune Tomato Plants: 7 Steps for Success

Can Tomato Seedlings with Purple Stems Recover?

The good news is that, as long as you fix any nutrient deficiencies, your seedlings will recover. This problem is quite common with indoor grown seedling, and you might have just a few seedlings with the issue or all of them.

Typically, by the time you transplant them into the garden, this problem is gone. You’ll also be pruning your tomato plants after planting them outside, so it’s possible to remove the purple leaves. Typically, they turn a normal shade of green as they recover.

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If your tomato seedlings have purple stems, go through the possible reasons for the troubles. Start with the most obvious and work your way through them. Luckily, the chances are your seedlings will survive and grow into thrive tomato plants before long!