Tuesday, March 25, 2008

How to grow healthy seedlings under lights

There were several questions asked on my last early tomato post so I thought I should expound on what has worked well for me. I have grown most of my vegetables and annuals from seed under lights for years. I have made many mistakes along the way and have developed a system that works well for me. There are a few simple tips that I have learned that might be helpful to you.

The five most often overlooked tips for growing successful seedlings:

1. Use a lot of light!

If you think your seedlings aren't getting enough light, you're right. And if you think they have enough light, they could still use more. With florescent light fixtures I am trying to mimic the bright and powerful sun, so the more the merrier. I like to have two fixtures per flat (four 40 watt bulbs). I don't use special light spectrum grow bulbs. They are an unnecessary expense. Although it is important to use new bulbs each year because the light bulbs lose some of their brightness as they age. I keep the lights on between 16 and 18 hours per day.

2. Keep the plants close to the lights!

I make sure the light fixtures are only an inch or so above the top of the plants. Remember, we are trying to give them the power of the sun, not the moon. Don't think in terms of how much light we need. I would rather err on the side of the plants touching the bulbs than too far away from them. Being too far from the bulbs causes tall spindly plants.

3. Use good growing medium!

I use the soil-less seed starting mix in the beginning and transplant up to a mixture of potting soil and peat moss. Do not use straight potting soil because it doesn't hold water well enough. You need the peat to keep the roots constantly watered.

4. Give them plenty to drink with bottom watering!

All of my containers have holes in the bottoms to allow water in (or out). Peat pots work the best for this. I put the cell packs, newspaper pots or peat pots in a plant tray that will hold water. Then I fill the tray up about half way and the porous soil medium wicks up the water to feed the roots. I water almost every day. I am convinced that this is where the most mistakes are made with growing seedlings. Constant light can dry out the soil quickly which stresses the plants. If you try to spray your plants from above, they probably don't get enough water. This is why I am struggling with using soil blocks. Since they aren't in a pot you can't bottom water them. I haven't used them for my early tomatoes. I will try again with the main season tomatoes.

Incidentally, making sure my plants are watered frequently enough is why I don't use automatic timers to turn the lights on and off. If I have to do it manually I am more able to monitor the soil moisture.

5. Get your timing right!

Make sure you know when you plan to move the plants outdoors and the growth habit of each kind of plant you are raising. Research the recommended seed-to-transplant time for each vegetable or flower. For instance, tomatoes should only be under lights for 4 to 6 weeks before transplanting and you shouldn't transplant them until all danger of frost has passed. In my area, the average last frost is May 15th. That means that I should wait to plant the seeds of my 27 main season tomato varieties until mid-April. If timed correctly, you do not need to pot-up very often. Of course for my extra-early tomatoes I transplant them two or three times and have them under lights for 8 to 10 weeks but that is unusual. For most of my flower and vegetable seeds, I try to wait as long as possible. Real-estate under grow-lights is at a premium, even for a big bench like mine. Be careful not to take up unnecessary space by planting too early.

Now, If you have never tried starting your own seeds indoors with florescent lights, I hope posts like these don't make it look too complicated. I love managing the plants under lights. For me it is great fun. It is especially rewarding because I know that I can truly raise organic vegetables. I get to control the plants' environment from start to finish. It is also fun to grow seedlings because you get a chance to get your hands dirty when it is still too cold to dig in the outdoor garden. I would recommend any gardener give seed-starting a try.



Thanks so much for this informative post. I'm growing under lights for the first time. I think I need two more bulbs :( I'm relieved to see that I'm mostly getting it right though!

You just rescued me from starting my tomatoes *way* too early. Our last frost date is near the end of May and I was considering starting the seeds this week.


Back in the 20s today in the Northland...and still a patchy covering of snow. Come on spring!!

What a delightful and useful post! Like you, I gave up windowsill growing for starting seedlings under shop lights long ago. What a difference! and you're right about the organic benefits. There's nothing quite like the satisfaction of knowing everything your food has come into contact with from the start.

And I learned something. I was only replacing shop lights as they burned out. So thanks for that important tidbit!


Amy, in regards to the two more bulbs, i am in the same page as you. I thought one lighting fixture was enough for what i was growing. I found out later than my trays are 12" wide and my lights about 8" BIG DIFFERENCE!.

I just could not efford to buy one more lighting fixture and two more growing lights. Its about $20 for the lights and $24 for the fixture.

So i ended up using alunminum foil to cover the plants over the lights. It help drastically because it reflects the light lost in space back to the plants. I did had to raise the lighting fixture a little bit more. I had it at 1" from the plans, now it is about 3". I have a feeling that at some point before planting outside, i am going to have to buy another light. Hope not!.

By the why Marc, when you get a chance, can you talk about feeding the seedlings. When should we start to feed them and how much. I read that it is recommended to use 1/4 or 1/2 of the recommended solution. This is confusing!. How do you translate 1 tablespoon of organic fertilizer (bone /blood meal, and fish emulsion)into 2 sqft of soil compare to diluting it in water?.

I am thinking of diluting it into the water before i added to the bottom tray, but.... how much fertilizer to a 2 liter soda bottle full of warm water?.

I am using compost, so maybe with this store bought compost is not required to feed the seedlings. Or is it?


Ahh, now I really like the idea of growing single lettuce under lights in the Winter! Suppose I could grow radish too? I get so impatient this time of year waiting for everything to grow!


Johnnatan - thanks for the tip about the aluminum foil. I was actually considering trying this so it's good to hear that it works

Kimberley @ The Life of A Garden

Oh god! Oh god!! I'm in such a panic. I KNOW I don't have enough light system for all the seeds I'm trying to start. Now I think I started everything a month too early because our last frost date is May as well. Already they're spindly. I'm considering starting over.
I have a faerie ornament in my kitchen that says, "If you're not killing plants, you're not stretching yourself as a gardener."
But I don't want to kill ALL the plants!


Don't panic Kimberley. You can still use the tomatoes that you already started if you have room for them. If not, then you can start new ones. But look on the bright side - you have already been gardening. For most of us, eating the produce is great, but the process is fun too. If you decide to re-plant, you get to garden some more. You faerie is right. Even if you kill your existing plants, don't think of it like you would for pets. I purposely start more than double the amount of plants I need and select only the best looking ones for the actual garden. The others go to the compost pile! No matter what you decide, another light fixture would be great if you have the room.

I buy the cheapest fixtures out there and ordinary 40 watt bulbs - usually about $15 total.

Johnnathan, you are probably buying better fixtures than I am - which is likely a good idea since they will last longer, but you should be able to find cheaper bulbs. As for feeding the seedlings, I usually don't. I think it is a good idea but I'm no authority on how much to feed them. I suggest an organic means (like compost tea or Great Big Plants) over a chemical fertilizer. I would rather transplant them, giving them new nutrients in the additional soil.
Good idea with the aluminum foil. With my first light set up I fashioned aluminum foil walls beside the trays for even more reflectivity. I should think about that again.

Matron, you are right that lettuce could be grown in the winter under lights. The flat of lettuce pictured has gotten larger than I wanted. Most of them are now outside, but we sacrificed some of them for salad already. The leaves were small but tasty!

Good luck to you all as well as Amy and Exuberantlady too!


you are right, i did buy the light for durability. Thanks for letting me know i can use the cheaper bulbs. If i have to buy another lighting fixture i might buy the cheaper one since they run for about $9 to $12 dollars. That is if i have to.

In regards to the ferlilizer, thanks for the input. I am not concern anymore since i transplanted with compost and that is why my plants for now are doing better.

I just don't know how long until they run out of nutrients but time will tell.

Marc, how do you control the temperature in your cold frame when it starts to get hot in April. Nights here in April are about 35-45* and days between 45-65*. At 65* my cold frame greenhouse can cook the plans and dry the pot soil. I am very inexperience with this.


You just saved me a lot of money, Marc! I'm a dabbler, and a complete novice when it comes to the finer nuances of gardening. Last year I planted out my plants as seeds right in the garden. although my pepper plants did very well, I'm just fantasizing now about how much better they can be if I get a headstart by growing them indoors. I thought I *MUST* use growlights for any indoor gardening. Hearing from you that normal bulbs work, I decided to try it.

I went out to my local home depot and picked up their cheapest 4 ft. fixture for $8.50 and two cool 40 watt 4ft T12 bulbs for $4.00. That's $12.50 when I was anticipating $60 and up. Thank you! :)


Thanks for this great post!
We are growing from seed for the first time this Spring and are at the moment designing and purchasing materials for our first seed growing rack.

Now I was wondering: what is the maximum height clearance for the average and the highest seedling, between surface of the soil and the lamp?

I've found lots of information on plants and seedlings etc. but not this particular piece of data. We need to know at which intervals to place the shelves on our rack.


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