Seedling Care – High Intensity Discharge Lamps

These are a viable alternative to fluorescent lights but I have never used them for reasons outlined in the Cost Considerations section below. Thus, the information provided here is from my research into these lights and not on personal experience.

These lamps come in two flavors: High Pressure Sodium [HPS] and Metal Halide [MH]. Both these lamps come in sizes from 150 watts to 1000 watts each though most gardeners tend to use the 400 watt version. The MH lamp puts out a wider spectrum than the HPS though the HPS puts out more lumens per watt. Also both require a ballast to operate.

These lights are expensive. A 400 watt bulb is $35.00 and the ballast/fixture is about $100.00 for MH and $200.00 for HPS. One MH lamp puts out 36,000 lumens, one HPS lamp puts out 50,000 lumens. This is enough light to illuminate a circle six feet in diameter at 1000 lumens per square foot.

In order to let the light spread out to a 6 foot diameter circle the light is mounted 4 to 6 feet up from the plants. This is an advantage in that the lamp doesn’t have to be raised/lowered like the fluorescent setup for watering and other seedling maintenance.

Incandescent Lighting

Incandescent lights include the ordinary screw in light bulbs that almost everyone uses in there home. They also include the tungsten halogen bulbs as well. There are actually companies selling incandescent “grow” lights. Don’t waste your money. These are horribly inefficient in terms of lumens per watt and will not put out enough light to grow pepper seedlings.

Cost Considerations for above lighting methods

After the initial outlay of cash for your lighting system there are other cost tradeoffs to consider. They fall into three general categories as follows:

Efficiency:

Lights convert the electrical energy into light energy. Some systems do this with less waste (heat) than others. Thus for a given light requirement your electric bill will vary based on the system chosen.

Below is a table which shows how many lumens per watt each system will deliver. A higher number is better.

Bulb Average Life Span:

Each of the different bulbs last a different amount of time. The table below rates each bulb for average life span. With fluorescent lights the numbers are misleading. The bulbs will last a very long time but their light output drops significantly over that life span. To maintain better than 1000 lumens per square foot I change the light bulbs every season.

I do not know if the Metal Halide or High Pressure Sodium lamps have a similar falloff in light versus time. I suspect they might since the spec sheets for these rate the light output under the heading “Initial Lumens” (the same as with fluorescent).

Lighting method: Lumens per Watt: Expected Life (hours):
Incandescent (Note 1)   17     750
Tungsten Halogen   21    2000
Fluorescent   75  15000
Metal Halide   90  20000
High Pressure Sodium  125  24000

Note 1: There are several incandescent bulbs that advertise themselves as “long life”, with up to 2000 hours average life. This comes at a price though. The lumens per watt number drops significantly with these bulbs, down to around 13 lumens per watt.

Incremental Costs

When the first seedling sprouts it goes out under the lights. It will be many more weeks before enough seedlings have sprouted to fill up the available space under my 8 shop light fixtures. With only a few seedlings I can start up only one shop light leaving the others off. As more seedlings emerge I can incrementally scale up by turning on more fixtures as needed.

With a Metal Halide or High Pressure Sodium lamp you get all 36000 lumens or none at all. This forces you to waste a lot of power at the beginning of the seedling process, and also at the end when just a few stragglers are left.

Also consider what happens when either a lamp or a fixture fails. With the fluorescent setup I can lose at most only 1/8 of the total light if a fixture fails or 1/16 of the total light if a bulb fails. Since both of these are readily available at any of several hardware stores, I don’t have to stock spare parts. I am also not so much in a hurry to replace a failed part since the light is down only 6 or 12%.

But what if the metal halide bulb fails? Or the ballast? This requires immediate attention since the plants are sitting in the dark. Can you get ballasts and/or bulbs in a timely fashion? There aren’t as many local sources for these parts. And if they are out of stock? You could keep your own spares but this ties up a lot of money.

What you have in your mind?