Why is my cilantro turning a yellow purple color?

Question

This cilantro was planted from seed using Pro-Mix seed starting mix on February 22 and the photo was taken today, March 28; 5 weeks later.

The pot is under timed T5 florescents (16h on, 8h off) and gets watered every 2-3 days as the soil dries.

I'm new to gardening and don't know why my once flourishing, vibrant green plants are now starting to slow down, turn yellow and then purple. I've read that if the plants don't get enough light or get too hot they can prematurely bolt, but these plants don't look overly crowded to me.

Any ideas?

Answer 1

Your plant looks to be perfectly healthy. However, check the soil and the drainage if the plant is getting too wet then yellowing leaves could be a sign of overwatering. On the other hand, you have soil fluctuations in soil PH and over fertilization could produce the same effects on the plant which leads to root rot. If I was in your position I would let it go for a week and if it continues to yellow then repot the plant. In regular potting soil or make your own potting soil.

Answer 2

I suspect your seedlings need nutrition.

Let me explain in a little more detail as to my reasons why...

I’m not familiar with “Pro-Mix seed starting mix” (seems that it is a US product), so I looked up the company website for the product. It claims...

Organic fertilizer that gradually feeds plants for up to 3 months

Once seedlings progress beyond their cotyledon stage - that is, they commence growing leaves other than the first two leaves that are contained in each seed - they have exhausted their seed-based food supply and require nutrition from their growing environment.

The seed starting mix contains an organic fertiliser that is claimed to provide food for up to three months, but we don’t know the amount of fertiliser or actually how long this will last.

As most seedlings in most human-controlled growing situations do not remain in their original sprouting media for long, it seems most likely that the included fertiliser is adequate to provide a little nutrition for sparsely planted seedlings.

Your planting however is very intensive - a lot of seedlings growing very close together. While this is not the issue, it’s important to recognise that these seedlings require the same nutrition (or actually more due to competition) as if they were spread out in a larger area.

I suspect your seedlings have exhausted the fertiliser contained in the original growing media and require an additional level of care that now includes fertiliser as well as water.

My recommendation...

To your watering can, once a fortnight, at half the manufacturers recommended dosage, add:

• liquid soil bacteria to inoculate the soil with beneficial microbiology;
• liquid seaweed extract to feed the soil bacteria;
• liquid organic fertiliser to feed the plants.

I have favourite products that I use regularly. While it is unlikely these are available in the USA, there will be similar products available in your area.

(As an aside, I note that worm castings - “vermicaste” - is an excellent alternative to this recipe, however I’m making an assumption that, as a new gardener, you don’t have access to a worm farm.)

Any questions feel free to ask in the comments and I’ll update my answer.

Answer 3

As it turns out, it seems the setup I had with the lights I'm using (2x SunBlaster T5HO) was a bit too intense for these plants. I moved the pot out to a West facing window that gets late afternoon/evening light, and within the first day I noticed improvement.

The plants have straightened up considerably and are starting to get their nice green color back. This is how it look after 5 days:

I also took andrewbuilder's advice and gave these guys some of the 1/4 strength 18-18-21 feed I had mixed for my other plants.

My problem, I believe, is based on my original hunch that the plants were bolting prematurely due to the excess heat from the close lights. Had I raised my lights to about 4"-8" my setup would have likely worked, but I need them lower for other plants so moving the cilantro pot to a cooler, less intensely lit area was a better alternative.