I have sprouted 3 Key Lime seedlings, they are in a perfect start sponge (1.5 ") and about 3.5 " total size (with root coming out bottom of sponge).

Is a 3.5" x 3.5" x 4" (height) container too small for first transplant?

I did need to peel the first layer of the seed coat to get a successful germination but 3 for 3 using a small plastic sundae container on a heat mat. Germination was 0% successful using a hydrofarm station (drying out of sponge was a problem even with daily sprayings).

I suggest 9 or 12 cm (3.5" or ~5") pot.
Then I would increase the pot size like this: 12 cm -> 15 cm -> 19 cm -> 24 cm and so on... (Repotting every year)

A 3.5 inch container is good but I would use a 4 to 5 inch size. Repot after the rootball has filled the container.

I took a chance and repotted some seedlings from 5 inch pots to 3 gallons and one survived and is growing but this is risky. It would have been better to use a smaller container. With containers too large relative to the size of the plant you must be precise with watering because too much water can kill the seedlings.

Thanks,

I planted them into 5.5" and 6" pots as I didn't have anything between 3.5" and 5" pots. They seem like pretty small little seedlings in there, we'll see how they do. They are indoors right now and probably will move out for short periods if the weather is right but right now wind is too strong for them.

Now long would you normally wait before applying fertilizer? I'm guessing several months or longer.

I would be very careful with fertilizing young seedlings. Wait at least 6 months before applying fertilizer.

i agree with TRI. applying a fertilizer at the early stage of seedlings' development might easily burn the root system...

Many growers begin to fertilize citrus seedlings at 1/2 the recommended rate, using standard fertilizer formulations, when the young seedlings have developed their first two true leaves. Studies by Dr. Whitcomb and Dr. Bisher of Oklahoma University, showed slow-release fertilizers can, and should, be added directly into to the germinating mix at the time of planting the seed, to insure optimum growth of the young seedling. In early experiments to grow tree seedlings in containers, the seedlings were planted in a well drained growth medium, and after seedling emergence, fertilizer was applied. It was noted that in most cases, the seed would germinate and the seedling would cease growing until the fertilizer application. This led to studies to determine how soon, relative to seed germination, the fertilizer could be applied. Early work suggested that a slow release fertilizer such as Osmocote could be incorporated into the germinating medium prior to sticking cuttings with no detrimental effects to root initiation, but with a stimulating effect on subsequent growth. Based on these experiences and observations, the slow release fertilizer 18-6-12 Osmocote was incorporated into the medium for germinating tree seedlings at rates of 24 pounds per cubic yard (16 kg/cu.m) of mix. Only at the 12 pounds per cubic yard (11 kg/cu.m) rate of Osmocote was the fertilizer detrimental. In the final analysis it was deterred that adding Osmocote 18-6-12 into the germinating mix at 6 lbs/cu.yd. (3.6 kg/cu.m) significantly increased plant height, number of bud breaks, stem caliper, top weight and root weight of tree seedlings. Additional Osmocote did not increase seedling growth.
(Taken from Plant Production in Containers-II)

I always use 18-6-12 in my germinating mix on all of my seedlings, and have for years, or I have applied a 25-5-15 W/Trace Minerals at 1/2 strength at the development of the first two true leaves many times, to provide additional strong growth of the seedlings. - Millet (549-)

Fascinating stuff Millet, thanks.

A slow release fertilizer is much better and lower risk of burning your plants. I use Osmocote and very satisfied with the results. I have been very cautious with seedlings because I only have one survivor from last year. There are two problems I think doomed my seedlings: failure to acclimate the seedlings to sunlight before placing outdoors and citrus leaf miner damage. This year I applied a systemic insecticide to eliminate leaf miner problems and so far there is no leaf miner damage on this survivor. In fact, I have not noticed any leaf miner damage on any citrus plant this year! Maybe the drought killed the leaf miners?


To avoid sunlight damage, I have decided to grow my seedling germinated this year indoors. My survivor from last year is in full sun outdoors.

Evaldus, the slow release Osmocote fertilizer formulation 18-6-12 is especially manufactured to be used in germinating mixes, and is very safely use on seedling trees from the very first day of their lives. It is very broadly used by the commercial nursery industry. - Millet (548-)

In my estimation, the safety factor, with most any formulation of slow release fertilizer, increases greatly with mature trees. When used according to the label, slow release fertilizers are about the safest form of fertilizing there is. My guess would be a mistake in the magnitude of 4X+ would have to be made to cause much damage. I have personally never met, nor have I even ever heard of anyone that has damaged his/her tree using a slow release fertilizer. - Millet (648-)

It seems we have some interesting factors to consider - liquid vs dry fertilizer and slow release and normal release and how they can impact seedlings and burn roots.

As a note I have a hydroponic system that I have put seedlings into (not citrus but basil, carrot, etc). with a nutrient solution of approx. 1100 ppm. The hydroponic system I have is an ebb and flow so it "feeds" nutrient solution 15 min every hour. At first I was worried about feeding nutrient at that rate to seedlings but so far at least 10 seedlings have survived and done just fine. When I first started the hydroponic system every seedling was in the same state and I ramped up the nutrients slowly. So, from my observation with my hydroponic system as 1100 ppm nutrient solution doesn't damage seedlings and since its liquid fertilizer salts won't build up.

That isn't to say that using liquid fertilizer will always prevent salt buildup because I have seen some nutrient fall out of solution. But, it certainly does beg the question as to whether liquid fertilizer is safer and almost completely free of risk for root burn. I think however liquid fertilizer has some downsides - its gone almost right away, some claim its more difficult to calculate dosages . . .

I have been reading so many downsides of dry fertilizer though - and it seems you can't really control it. You could apply it and then have a heavy rain and it could all wash away . . .

I'm curious why more people don't use the liquid fertilizer like they sell for hydroponic crops? Is cost the main factor? Weight should be similar as the concentrated stuff works about 1 tsp per gallon.

What actually is "fertilizer burn"? Fertilizers are salts. These salts are not unlike table salt, sodium chloride, except that they contain various plant nutrients. When a salt is added to water, osmotic pressure of the solution is increased. For a plant's root system to take in water, the water must pass through a root cell membrane. Water can pass through this membrane ONLY when osmotic pressure of the solution inside the cell is higher than osmotic pressure of the solution outside the cell. Water moves from a solution with low osmotic pressure into a solution with higher osmotic pressure. If the osmotic pressure of the solution becomes higher (as a result of soluble fertilizer applied) than that of the solution inside the cell, water cannot enter the cell and, under severe conditions, may move out. This results in damage to, or death of, the cell. When root cells die, the whole plant is damaged or may die. The end result is termed "fertilizer burn".(taken from Plant Production In Containers-II) - Millet (547-)