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I was at a couple of different stores recently that were selling tomato plants and saw some more from another nursery that still had green and healthy looking cotelydons (seed leaves) on them even though the plants were about 5-6" tall with a lot of true leaves. Most were in 4 packs but some were in larger pots. Same with some plants I ordered from a large seed/plant supplier online last year.

When I start my tomatoes from seed the coteledons start to yellowafter about 3 weeks when they're still only about 3" tall or so with only 2 or three sets of true leaves. I started them in 98 cell trays with an organic soil that contains fertilizer and also fertilized them with a little dilute fish fertilizer a couple of times when they were still in the 98 cell inserts.

At 4 weeks old, when I transplanted them to larger pots, the cotyledons were practically falling off the plantson most of the plants when I just touched them with the scissor to cut them off so I can plant them deeper in the larger pots. It's been about 4 weeks since potting up and the plants look great.

Why are the cotyledon's on the plants at the nursery still completely green?

Here's a picture of my seedlings at 4 weeks. Sorry for the poor quality, taken with cell phone in dark room. enter image description here

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Likely its to do with nitrogen supplies - the cotyledons give up their nitrogen to the growing tips and leaves at the top if it is in short supply. Professional growers know exactly how much feed to give and exactly when, and may start their plants off in larger compost plugs initially anyway. It doesn't matter that yours are yellow, so long as the true leaves are growing away nicely.

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  • Thank you. I was worried about over fertilizing so I based it on the color of the true leaves (photo a little washed out from flash) not the cotyledons since I knew I'd be cutting them off anyway. Next year I might add a little more fertilizer but I'm also thinking it may be good that the plants got to use up everything in the cotyledons before I cut them off. Is that misguided thinking?
    – OrganicLawnDIY
    May 7, 2015 at 16:24
  • It's irrelevant whether that happens or not - but those yellowing cotyledons are most definitely a signal to pot them on into something with a bit more fertiliser! Difficult to get it right initially - too much fertliser and it burns the seedlings.
    – Bamboo
    May 7, 2015 at 16:56
  • It also has to do with the (insane) speed at which they grow them in nurseries, under optimal water/light/fertilizer/temperature levels, and often supplemental co2.
    – J. Musser
    May 8, 2015 at 1:11
  • This year I fertilized the seedlings more. I read a bunch of different production guides and fertilized according to the recommendations. Usually on the high side so if the recommendation was 70-100 ppm N I went with 100 ppm. Plants had ample light and temperatures were in the range. At around the same age the cotyledons began to fall off again. By the 4th week when I moved them from plug trays to pint containers the cotyledons were gone. @J.Musser any more info on the insane speed they're grown in nurseries? I suspected the plants might be younger since they still had their seed leaves.
    – OrganicLawnDIY
    May 6, 2016 at 20:07
  • @OrganicLawnDIY I've seen 4' tall monsters that still had healthy green cotyledons
    – J. Musser
    May 6, 2016 at 20:31

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