Would this work?

[estación seca]

Halide refers to high-pressure incandescent sodium lamps, not high-output T5HO. Halide is older technology. It works well, was the indoor marijuana growing standard for years, but has many drawbacks.

I would recommend you absolutely not use high-pressure sodium lamps in a closet. One good-sized lamp, together with its transformer, is enough to heat a basement in winter. I think they are too hot to be used in a home. They can give you a third-degree burn in a fraction of a second, before you have time to withdraw your hand.

High-output T5 fluorescent lamps (T5HO) have worked well for growing plants. They are still available. But LEDs are considered better from the standpoint of efficiency and temperature. I suspect adequate T5HO in a closet would heat the closet too warm for most plants.

There is a thread here devoted to growing under lights. From the left yellow menu, select Forums, then Growing Under Lights. Realize technology has advanced rapidly, and some of the older threads have been superseded. Some of the older threads contain excellent information. You will read posts from people who build their own fixtures, as well as recommendations from other people as to what commercially available fixtures work well for them.

[Thyroyalgeek]

T5 Halide meant both lights, sorry about the mistype. T5s as main, with halide as a supplement. But Now I think I'll probably just switch T5s for led, and possibly cut out halide, but I did hear it helped in addition, just not as main lighting. Is this correct?

[estación seca]

Lots of people here grow under only T5HOs with great results.

People often find a color temperature of 5,000-6,500 K works best for plants. The very bright aquarium reef lights of 10,000K don't work very well for plants. Those are for corals. They do make flower colors pop for human eyes!

It takes plenty of fixtures to grow high-light orchids like Vandas under T5s. The distance from tube to plant is very important; generally the closer the better for higher-light plants. That can lead to temperature problems. You can easily burn your finger on a T5 tube. Same with plants. Read through the lighting forum to get an idea how far from the tubes people put their plants. Many blow a fan across their plants to keep the heat down.

As I mentioned, I would never put a halide light into a closet, unless I wanted to burn down the house. In the old days I used a metal halide light in the basement of a St Louis house to overwinter cactus and succulents. Trust me, they are incredibly hot.

[Thyroyalgeek]

Thanks I think I'll try LEDs, but maybe I might try t5s out later. Last question, are 30 watt LEDs (full spectrum) put in a clamp light OK?

[WeirdGuySeattle]

Hmmm, a dug out greenhouse could probably work well. Lots of insulation, but agree it would have humidity and flooding issues - lots of fans, maybe a sump pump.

I've been intruiged by people putting airflow pipes deep underground their greenhouse and pushing air through them with a fan for cooling in summer and heating in winter. I am going to try it if I can find the right fans.

Seattle weather is pretty mild, so if 3degrees is going to be almost typical weather, I'd be concerned too. I'd definitely build a wood structure, with a few layers of insulation, both on the ground and dirt walls, including reflective stuff on the back wall.

rain barrel heat sinks would be good.
For a structure 20X15, you should look into installing nat. gas or propane heating (requires venting) for winters.

Summer time is also something to think about - how to cool it in the summer, but being underground I think you definitely get some insulation advantage.

[Leafmite]

Growing plants under lights works very well. I started with fluorescent lights for the orchids but now also use them for my tropical flowering/fruiting plants, geraniums, jasmines, citrus, figs, passiflora, etc. One year, I grew saffron and strawberries. Even full sun plants produced flowers and fruit.

We had another member who had a very nice closet set up so it can be successfully done. With LED lights instead of fluorescent lights, you won't need to worry as much about the heat.

I have no idea how many plants/orchids you are planning to grow in your closet and what your budget is but here are some possible items that might be of use:

Here is an example of LED lights:

Technical Data - SunBlaster Lighting

They have different sizes. How many lights you need per shelf will depend on what you grow. LED lights wouldn't get as hot as the fluorescent lights that I use. I use all fluorescent lights except for one two foot LED strip. If you have a hydroponics store, they can help you select the lights and ballasts. Make certain that the ballasts come with a way to hang them (mine has holes I can run the S-hooks through) and come from a reliable brand for safety.

These are great shelves:

Amazon.com: Alera ALESW604818BA Complete Wire Shelving Unit w/Caster, Four-Shelf, 48 x 18 x 72, Black Anthracite: Home & Kitchen

I attach my lights snug against the shelf above to give my plants the maximum space to grow. I use S hooks for the big four-foot, four lamp ballasts.

I would enclose all but the front of your shelves with 6 mil. plastic sheeting to prevent mold in the closet (including underneath). In addition, I line each shelf with $5 clear plastic table clothes that I cut to size to protect the lights on the shelf below. Between the shelves and the sheeting, you could even place reflective material to help distribute the light.

Using a timer makes it all so much easier. I have had this one about four years:

Century 7 Day Heavy Duty Digital Programmable Timer - Dual Outlet (Single Control) - - Amazon.com

I hate the directions for programming it but better directions can be found online.

Last of all, use a power strip that has a fuse and surge protection to protect your lights during power surges.

Pictured is a two-foot, single-bulb fluorescent light tucked under a shelf in a dark spot. The orchids and geraniums bloomed.

[Orchid Whisperer]

At the University of Georgia there is one Plant Sciences greenhouse that is partially below grade. I've no idea if it has a better or worse energy use profile than other adjacent greenhouses that are all above grade, though I suspect it is more energy efficient.