Cotyledons and True Leaves

I am growing plants from seeds for the first time. I have a 24-bin kit in which I planted 2 seeds each of different varieties of sunflowers and zinnias in coir pellets which I have keep by a sunny window indoors. After 3 days, stalks began to emerge and eventually they all had varying height stalks with a pair of leaves. It has been 17 days and now there is a now two pair of leaves (one pair higher than the other) on most of the stalks but they look identical to the original leaves. Are the lower pair of leaves the original cotyledons and the higher pair the true leaves or are they all cotyledons? From what I have read, the true leaves should look different than the cotyledons and when the true leaves appear, that is when I should start the hardening off process of gradually introducing the plants outside before outside planting.

My questions are:

How can I tell the difference between Cotyledons and True Leaves?

Is it really necessary to "harden off" with sunflowers or should I just plant them in the ground without the week of transition?

With sunflowers, would I probably have had just as much success planting the seeds directly in the ground outside instead of starting them indoors?

The initial stalks started out standing up but most are now laying down (but they are still green). Should I be supporting them with sticks so they point straight up or just let them grow as they want?

Thanks to anyone who can help me learn this seed growing process.

bkoopers. First off, good for you to start a new learning experience: sowing seeds.

The cotyledons are the first pair of leaves on a plant. The next leaves are true leaves.

Sunflower and zinnia seeds do really well in situ (right in the ground). You have to get the soil to a fine tilth first and the soil must be warmed by the sun to about 68*. Big seeds are easier to grow in situ.

Laying down stems is not a good sign, but plants want to live, so they may perk up if you get them outside in some good light.

I would put the plants outside in bright shade and away from the wind now. You might do a half a day for a day then, all day for a day, then, in the ground.


Gardening is a learning experience. I think they should have gone outside as soon as they came up unless the temps were too cool. Or, you probably could have put them outside in pots and let them sprout, then, put them where you wanted them in your garden, but the temperature outside must be warm. Again, "most" seeds sprout around 68 degrees. A lot of directions say "sow seeds outside after all danger of frost is gone".

I think I would start more seeds either in pots outside or in situ just in case these don't make it. Even if your plants don't make it, you have learned something. Sunflowers and zinnias are pretty tough. They may make it. I hope they do.

Seedlings started indoors need to be hardened off, regardless of species. Yes, in future, it is just as well to start most easy, fast-growing annuals outdoors directly where you want them to grow. When they germinate on their own outdoors, it seems they will generally be small enough or timed well enough to withstand or avoid any late frost that might occur (whereas purchased frost-tender bedding plants will often be damaged or killed).

Yes, the flopping stems is not a good sign... it may be inadequate light, not enough water(?), or possibly fungal disease (hopefully not).

I just set my indoor-grown seedling trays in the shade for 5 days or a week... shade outdoors has greater light intensity than sun through a window indoors. No need to haul them back and forth into and out of sun. (I'm all for simplicity! :-) ) It's worked with complete success for the large range of perennials I start each year (and annuals are no different with respect to hardening off).

This message was edited Jun 2, 2015 8:28 AM

By now you have probably figured this out - but the cotyledons (seed leaves) come out of the seed and are shaped like the seed, smaller and thicker than true leaves.
For lanky seedlings, I always assume it is okay to bury them up to the seed leaves when transplanting, and sometimes even make a little prop or crutch out of a twig. I have a better brighter light setup now & usually don't have lanky seedlings unless they outgrow their pots and get over-crowded.

>> sometimes even make a little prop or crutch out of a twig.

Someone suggested slitting a soda straw lengthwise, then traping the sedling stem inside it, to support the seedling.

I never tried that: it sounds like I would have to be coordinated.

>> I have a better brighter light setup now & usually don't have lanky seedlings unless they outgrow their pots and get over-crowded.

I agree 100%. Bright light plus pricking-out, potting-up or planting-out before they outgrow their seedling cells usually prevents "leggy" seedlings.