If the cell-count is 12 x 24, then you likely have 288s, which means each square would be about 0.75" x 0.75" by about 1.25" deep. Or, at least, that's what Dillen's 288s measure. Other manufacturers might use different sizing.

I only use plug flats that small for germination of small to medium (tomato or pepper-sized) seeds and then pot up to individual containers either as soon as the first true leaves appear or shortly thereafter. Some people pot up to larger cells or containers as soon as they have cotyledons.

When the cells are that small, the roots will get cramped quickly, so I think it is always best to pot up as soon as possible.

You could get away with leaving seedlings in the plugs longer for plants that start with teensy-tiny seeds like petunias or begonias because their early root systems are small and stay small for a while.

I wouldn't attemp to harden off anything in cells that small for fear they'll dry out too quickly. Those cells are tiny and we're usually hardening off plants in lots of windy weather that dries out small cells rather quickly.

Your mileage may vary.

Dawn

I sprouted a bunch of flowers in a 288 last year. Since they were "just flowers", I didn't care if I ended up killing them. LOL It was my first year using plug flats so I had no confidence.

They did great coming up and I even managed to not kill most of them from drying out. And, boy, do those suckers dry out fast, even without hot Oklahoma weather. But then, when it came time to transplant them out to their final in-ground homes (because, afterall, they were "just flowers" and not worthy of my time to pot up), I killed a good portion of those suckers trying to get them out.

You can pull some of them out as with 72-cell trays but it ain't easy, cheesy. Because the cells are so small, the sides are far more rigid, making the ol' method of a gentle crinkle & push fairly impossible. You can try to pull them up by the plants but, again, it's difficult. The roots crowd so quickly and/or the soil dries so quickly that it more often in not resulted in plant murder. There are small holes in the bottom of each plug through which and self-respecting gardener would try to push out the plug with a handy small-head nail or pencil. This last method worked the best but still ended with much grief.

I'm sure someone out there has some magical method for getting seedlings out of plug flats without all the death & dismemberment but it's not me. Phhhbbt.

Diane

I have used both the 512 and 288 cell flats in the past. I haven't in several years. Especially with the 512 and even the 288 they worked fine if I potted up no later than 5-7 days. That is what most of the commercial germination mixes of today are for. And why many greenhouse nurseries don't use them as much as in the past. I had a good visit with a lady in charge of the potting up at a large greenhouse in the TX Panhandle Friday. When I did use them I kept a narrow slender plastic knife, ect I could run down the sides. Usually one side was enough. Sometimes if I left them in the cells a little long I would have to go down all 4 sides. Then they would come out. With my work situation the last few years I haven't used them as you don't have much window on when to pot up. And last year especially I wasn't able to pot up when I needed too all the time. Using a standard potting mix like Ball or Pro-Mix you have a little more time in 72 cell flats. If needed you can leave them a few more days. Jay

Diane, I think that using a pencil to push the seedlings up from the bottom works as well to get the plants out of those tiny cells as anything else.

Jay, I have tomato seedlings from seeds sown on 1-21 that I need to start potting up tomorrow. Yikes! They sprouted quickly and have grown quickly, so I can't delay. I was hoping I'd be potting up on Super Bowl Sunday (my usual seed-sowing day) but I don't think these plants will wait that long.

All signs here continue to point towards an early spring warm-up. Our low temperature tonight is supposed to be 54 and our high tomorrow about 70 degrees. lol Down in Grapevine, TX, today, we saw willow trees leafing out. Tim said he first noticed them last week, and in our front yard, dandelions are blooming. The plum tree blooms are starting to show a little petal color.

I love spring, but I am not ready for it to arrive this early. I have a new winter coat I haven't even worn because it is a heavy coat and we haven't been cold enough for me to wear it.

So, for those of you who haven't sown your seeds yet...I'd say the time is growing near. I think we're going to have an early spring warm up, and I am determined I'll have tomato plants ready to go into the ground when the weather allows.

Dawn

I am set to start Tomatoes (6 types), eggplants (3) and Chilli (3), some greens tonight and tomorrow morning. I started Snap Peas in the weekend. Few raised beds are ready to plant, hoophouse will up once I plant those beds with radish, peas, spinach, funugreek, cilantro etc.

Dawn, Diane and Jay,

Thanks for the feedback. Your statements pretty much confirmed my intuition, dashing away forever any hopes of beating the system, she said dramatically.

Dawn and Chandra,

I've been so pleased with my progress being prepared for the season. I'm just prepared enough that I don't feel panicked and behind. However, if you are starting already, then I am in fact behind, and therefore panicked. AAAAAhhhhgggh!

I had a gut feeling on the early warmth, which is why I chose and onion delivery date on the early side of reasonable. Now that you guys are going early too, if this onion thing goes awry and we get 15 degree weather in April, I can point the finger at you and say well they did it too!

My uncle planted onions and potatoes 2 1/2 weeks ago in Norman. Yep, I'm behind.....

Jay, Are you saying I will be okay using Pro Mix BX in my flats? I have a new bag of Jiffy Mix, though I've not used their stuff in 25 years, but no seed-starting-specific Pro Mix.

If you are behind, then I am WAY behind. I have no onions out! Not even the seeds! I have been scared that the minute I go get everything ready for an early spring *BAMMO* a huge winter storm will hit and wipe out all the hard work. LOL

I need to get on the ball.

I germinate and I see green leaves of any kind in my plug trays, then pot up in their permanent pot until they go out.I will say this, I have been using coconut coir and they are getting to that point so fast in that stuff! I love it!

Seedmama I used to use germination mixes more than I do now. I will start most if not all of my tomatoes and peppers in the regular potting mixes I use. I was talking to the lady who is in charge of potting up at a large greenhouse in Amarillo last Friday and she stated they normally use the same mix to start that they use to pot up and grow in. Only on seeds that require a very light mix do they use a germination mix. I have started all of my onions in Ball and Berger potting mixes with great germination rates. I have used the Jiffy seed starting mix before with good results. But personally don't see any benefit to buying a germination/seed starting mix which is usually higher when there is no benefit to it. Jay

Seedmama, Sorry we dashed your hopes of beating the system.

I don't think you're behind. I just think we're trying to get ahead a little bit.

I am worried about a snowstorm in March, but that's the advantage to starting my own seeds. I start 3 or 4 times as many as I'll need, and I start enough to have a back-up for every plant that goes into the ground. So, if the whole garden freezes, I have backups and, sometimes, even backups for the backups.

I will stress that I am not going to put anuything in the ground that I cannot cover up.

I have germinated seeds in coir pellets, peat pellets, and Jiffy Mix this spring so far, and have equally good results with any. I don't think seed-starting mixes are strictly required. I tend to save the Jiffy seed-starting mix for petunias and other tiny seeds, and I mean the really tiny ones. Everything else germinates and grows fine in just whatever I have handy.

I am trying to get everything potted up quickly because if I start getting behind on potting up tiny plants, I never seem to catch up. I hope to pot up about 400 tomato plants in the next two days.

My goal is to start putting tomato plants in the ground sometime in the first two weeks of March, about 2-3 weeks before my average last frost date. I am going to wrap the cages in frost blanket material to give them a slightly warmer environment. I'll leave the top open, but can walk down a row and put a row cover over the top of all the tomato plants if we're going to go down close to freezing temperatures.

I hope to get onions in the ground tomorrow morning. I'd hoped to have them planted by now (yes, also early) but we had 3.3" of rain last week which has set back my plans a bit. It really is too wet to plant the onions in 98% of my garden, but they're going into a raised bed on the highest ground available and that bed drains well so I think they'll be OK. More rain is in the forecast for this week, so if I am going to get them in, I need to do it now.

Dawn

The only thing I have planted are the onions and leeks which are inside on the light shelf.

I plant tomatoes every year in small blocks that are three quarter inch cubes of seed starting mix. I normally don't move them until they have 2 real leaves which happens fairly quickly. The roots are always through that block and into the blocks next to that one by the time they have those 2 leaves. I can easily pick them up by the block and give a little tug to pull the roots out of the other blocks.

In years past, I had to move that little block to a small pot or cup at that time. I have other block makers that are nice for bigger seeds. The part that I bought this year is an insert that is a 3/4 inch cube that fits inside the larger block maker as you are making the blocks and creates the block with the 3/4 inch square hole in the top of the block.

This year I will be able to plant in the small 3/4 inch block, and anytime after germination, I can just drop that block into the hole in the larger block for the second potting up. The next time around, it will get buried a little deeper into a real pot where it will stay until the weather is ready for it.

I have attached a link at Johnny's for the soil blockers. The top item is the larger blocker and just below is the insert set that makes it leave a 3/4 inch hole in top when the block is made. Down the page, the red Hand-held 20 Soil Blocker is the one that makes the first block that I actually plant the seed into.

If there was an insert available when I bought my soil blockers, I was not aware of it, but I have had my blockers for years and years. There is a learning curve to making the blocks, but once you master it, they go pretty fast.

With the small blocks, you don't have to be as gentle as you do with the 'baggie' method, and if you are a few days late in getting them potted up, it's not a loss cause.

Here is a link that might be useful: Soil Blocker

I use soil blocks, too, and have had some great success with them. I just have the 2" blocker, and tomatoes and peppers start in them, and stay in them until they go in the ground. The roots do spread to the neighboring blocks, but that's never been a problem. I'm going to use coir in my mix instead of peat this year. I expect it'll work okay, but that remains to be seen. I've have mine for years, too, and love it!