I hate to be negative, but...Garden greenhouses can be very expensive to operate, especially with rising fuel costs. You will need some kind of automatic system for cooling and heating, since temperatures change very fast in a closed greenhouse. The commercial guys have everything under computer control. In the summer, you will need shade cloth and maybe a misting system. If you use mist, you better make sure that the greenhouse benches, frame, etc. are made of rot-resistant materials. If you lose electrical power during a blackout in extreme hot or cold weather, you could lose everything in the greenhouse. Are you sure you want a greenhouse?

I raise my seedlings under fluorescent lights in the house. This gives me easy access, since I like to watch them grow, and the heat from the fluorescents helps to heat my house. The best place is in a cool area (60-65F). If the room is too hot, you may have problems with germination of certain species, or the seedlings may be weak and straggly.

If you are growing orchids in a greenhouse, your energy costs will be enormous both summer and winter in Sacramento. If you are growing epiphyllums, you might consider overwintering them in a frost-free greenhouse, and moving them to a lath house for the summer.

It depends on what you plan to grow and when. Some plants have high lighting requirements, some don't like excessive heat. Most of the plants people grow in Sacramento will do fine with the heat, and remember, people use greenhouses in other warm areas of the country (FL, GA, TX, etc) and do fine dealing with the heat. You will have to keep it opened up and ventilated though. Some growers here use evaporative coolers to keep temperatures down. Some have greenhouses with each end opened up with one end facing the prevailing winds, then put the ends back on when the weather gets cold. If you decide to start growing something during the winter (seeds, cuttings) you can make a small enclosure to heat within the greenhouse instead of heating the whole greenhouse, or use a heating mat. Most winters here bottom out in the mid-20's, so keeping the greenhouse warm enough to overwinter plants shouldn't require much heating unless we get a major freeze.

I would think dappled shade would be acceptable and keep temperatures down at the same time.

You may consider getting a pop-up greenhouse, which is the kind I have, so that you can put it up when it gets cold and take it down when it gets hot. I have a pop-up greenhouse that you can walk into, not a little lean-to greenhouse or deck greenhouse. You just need to attach cables to it and stake it in the ground to prevent it from blowing away.

Yes... blowing away... mine did that! :o)Last year... during that windstorm, if anyone remembers.
Depending on what you want to grow, I have a friend who just gets those propane canisters from Home Depot and just heats it when it gets down to freezing... Plants that can go down to 40 will do fine with that, because the greenhouse will hold some heat from the sun.
I put mine in full sun, so that the winter sun, whatever there is, will partially warm the green house, but then you have to put shade cloth over it,during the summer, unless you have UV Glass.
Depending where you are in Sac, many things will do fine with just frost covering... like mangos and papayas and guavas.... If you want tropicals that are less hardy, you will need to spend $$ on heat. A friend of mine in Livermore used one of those "radiator" Delonghii heaters...
I just keep hoping we have a warm winter... but this fall is foreboding, huh!

You can use large water tanks to store and absorb heat during the day and it will be released slowly during the night, it helps a lot during winter, it stabilizes your greenhouse temperature. During winter, you can attach reflectors in the north wall of your greenhouse so that the lights reflect back into the water tanks of the greenhouse, then you will always get your greenhouse temp above 40 the entire winter without a dime of electricity or gas, most of the time around the temperature will stay above 50 if you have no major air leak.

You said your main use will be for growing from seed in the winter. I use mine mostly for propagation and have shade from tall trees to the south. This gives me shade in the winter when the sun is low in the sky. This is the time of year when I need the most light not the least. I must run a 400 watt lamp to grow good seedlings. I use a bottom heater to maintain 70 degrees soil temperature for germination. In the summer the sun is high and keeping the greenhouse cool enough is a problem, but I find I don't need it in the summer for propagation. A screened lath house will be fine for summer time propagation. If I had the choice I would pick the full sun location to benefit from the winter sun. Seed germination temperatures are easily maintained with bottom heat and after germination cooler temperatures are OK. To keep the whole greenhouse 65 degrees in the winter is a considerable expense. Al

Thanks for the input. I can tell I have a lot to learn ... and think about.

I've had two greenhouses. Now I'm not sure I could live without one. I also just assembled a Rion for a friend. Can't say that I'd recommend it. Costco imports lots of them from Israel and sells them cheap, but the glazing is VERY thin, and the PVC frame won't last long in the sun (it isn't very weather tight, either). There is also no exhaust fan or intake shutter. Don't be fooled by those solar-activated vent openers. Might be okay if you live in coastal England, but not the Sacramento Valley.
You might consider building one using opalescent twinwall on the roof, and clear twinwall for the sides. Even with shade cloth and misters, your exhaust fan will run all day long in the summer. You might need an evaporative cooler.
A simple poly tunnel where the plastic can be removed and replaced with shade cloth in the summer might be a better introduction to greenhouse growing in your area.

Dick Sonia, can you give me the cheap sources of your twinwalls? I'm tempted to contract out a glass house built next to the house (estimated cost $25,000), or maybe cheap twinwalls, if I can get the source, will build it myself

I live in the foothills east of Sacramento (temps down in the 20s in winter) and tried the winter sowing method 2 years ago. Wow!! I'm a convert!! Start saving your milkjugs and takeout containers, have a seed sowing parting on the winter solstice-ish, leave the tops off the neck of the jugs, poke holes in the tops and bottoms of the takeout containers, and don't worry about your seeds til it starts to warm up!!! I've gotten so many almost free plants by doing this. See the Winter Sowing forum for FAQ!

I live in Sacramento and bought a greenhouse in 2004 from Farmtek similar to the Rion model. I am still in the process of assembling it on my new property where Iam relocating.
Folks do have them set up out here. Logically it is best to determine what you are planning on growing and judge whether the use of heating or cooling is needed. Ideally I am selecting a location that receives sun in winter and shade in summer. If you live in a residential neighborhood the side of your home between two houses is a good place especially if it is next to a two story. I have a two story home and live next to two of them on each side, so I get sun but not enough in summer to cook the plants.
Winters in recent years have been very mild so you may not even have to resort to providing winter heat, but then again it depends on what you are growing like South American tropicals or something. I have seen greenhouses here with winter heat and no heat.
I would set up the greenhouse and play it by ear. Frost is the big killer of plants and by having the greenhouse you have shielded plants from it. Most of the time frost sets in and it only lasts for a few hours anyway on the valley floor and it is gone anyway.
Last year I had tomato plants outside in January and they did fine. At night I just moved them onto the porch with my avocado trees in containers and they did well. If they can survive there, they should do well in the greenhouse. I even had a volunteer Giant Greystripe sunflower germminate in February and by June it grew into the largest sunflower I had ever grown reaching almost 2 ft across. Happy gardening

I know this is an old link... did you ever buy a greenhouse? If not, you might want to read on. If so, what has been your experience?

I have just finished reading an entire book about building and managing your own greenhouse (Rodale Press, available at Sac County Library, www.saclib.org GREAT online library services!). As a result of this reading, I have decided not to do greenhouses. They are expensive, if done right and to last, and require a great deal of work and expense to maintain, heat and cool.

In addition, the winters here (Sacramento, valley floor) are mild enough that many crops, planted in July, August and September, will grow and produce throughout the winter. A sunny window sill is all you need to start seeds for your spring crop early. Unless you are growing rare tropicals that you simply "must have," I agree with LilyDude: there are other, less expensive options that will suit your needs.

If you have not looked into these options, read up on:

Cold frames--easily built from salvaged lumber and old windows. Check the Habitat for Humanity Restore (at the end of North Seventh Street, off Richards Blvd. http://www.shfh.org/restore/) for cheap, double paned windows and other new and used building materials.

Hoop Shelters: Mel Bartholomew, author of "Square Foot Gardening" has developed an easy-to-make hoop frame system using 1x4 lumber and chicken wire that can be covered, interchangebly, with clear vinyl, bird netting, or shade cloth, depending on what is needed at any given time. Plus, they are cheap to make, lightweight and portable. With a few modifications, you can even build them to break down easily for storage in the attic when you're not using them. Who could ask for more?

Even cheaper, the old milk jugs and water bottles, placed over your seedlings (these are called cloches) are the cheapest way to go, and very effective. However, for a better germination rate, I would recommend starting your seeds indoors and transplanting after they have at least one set of true leaves (2-4" is the ideal size for transplanting), then cover them with the cloches. The heat generated during the day and retained at night by the cloches might not be enough for them to germinate reliably.

NOTES: Reflective insulation is a good idea, but regarding using water bottles to collect heat during the day and release throughout the night: this is certainly done by many greenhouse gardeners, and is not "wrong;" however, Rodale does not recommend this, as the water mass to be heated competes with the plants for heat. In Sacramento, with our mild winters, this would probably not be a huge concern, but worth mentioning.

A better use of water for retaining heat, in my opinion: the WalloWater is great for Sacramento's hot summers. They collect heat in early Spring to get seedlings established, then work night and day during the summer: cooling the plant's roots throughout the blasting summer days and then releasing the heat throughout the night, for a more stable 24/7 temperature. They are available at Capital Nursery in Elk Grove and on Freeport Blvd. www.wallowater.com