If you had your seeds started indoors in late winter you will have a load of vegetable seedlings to care for by March. We have a load of tomatoes, cucumbers, zucchini, and lettuce started indoors right now.
The heat-loving plants e.g. tomatoes will have to stay indoors until the weather is warm enough for them in our polytunnels or outside. With us, that is usually around mid to late April and if you live in colder areas it might be mid-May. The seedlings will need good care until then to have strong plants that will grow healthily.
You want to avoid conditions that will make the plants leggy and starved because they will be slower in their development later on.
Poor quality seedlings will also be more prone to pests and diseases. Instead of getting established quickly these plants will take several weeks to recover before they start growing and producing.
Once the seeds have germinated all you have to do is keep them moist...but not wet! I prefer to do the watering with a spray bottle every couple of days because the fine mist doesn't harm the tender young seedlings. Make sure not to overdo the watering particularly at this early stage. The plants won't need a lot of water yet and if they are kept too wet they will get affected by a fungal disease. This is called 'damping off'. The seedlings basically collapse and die. No feeding is necessary at this point.
Also important is to keep them in a bright spot so they don't go tall and leggy. If you don't have good light for your seedlings you can use grow lights. These have the right spectrum of light for plants which is different to ordinary light bulbs.
Seedlings that are kept indoors until the weather is warm enough for them like tomatoes might outgrow their small pots, plugs or seed trays after a few weeks. Once you see roots coming out at the bottom of the pot and the seedlings have set the second set of leaves it is time to transplant them into bigger pots.
We always add some organic fertilizer and seaweed powder for extra minerals to the potting soil. This will keep the seedlings growing healthily and vigorously and we don't have to worry about adding a liquid feed to the water for 5-6 weeks.
If you don't do this you will have to use a liquid organic fertilizer once a week. I would also recommend using liquid seaweed because it really helps the seedlings to grow a good root system and to be disease resistant.
A week or two before planting the seedlings out you should harden them off. This basically means that you have to get them used to outdoor conditions like sunlight, a breeze, and cooler temperatures. Bring your seedlings out for a few hours a day. Choose a sheltered spot that is a bit shaded so that your tender seedlings don't get burned from the sun. I know that this step sounds a bit labor intensive because you have to carry everything in and out for several days but you could risk all your hard work if you don't harden them off.
I have to admit that we often skip this step because we have just too many plants and seedlings. Instead, we use garden fleece to protect the young plants from too much sunshine and cool temperatures. I explain the benefits of using fleece here.
You can also use a cold frame or hoop tunnels for hardening off the seedlings.
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