I was asked to summarize what I’ve learned about growing the best possible fruit in a greenhouse. I’ve been at it 20 years in Alpine Texas at 4500 ft elevation. It’s sunny here year around, ~75% sunshine all year long. 60/30F in winter, 90/65 in summer. Humidity is generally low especially in winter but can be higher than desired.
Growing fruit in a greenhouse is pretty challenging even here in a good environment for greenhouse growing. What’s a good environment: warm, sunny, and dry. The same as outdoors. Right now we’re having dew points between 55 and 65. That works and I can keep the greenhouse cool enough using a wet wall and large exhaust fans. But when it turns cloudy and rainy, fruit quality of the most sensitive fruits like figs suffers. For one thing the figs are rooted outside the GH so runoff from the roof affects them. I have up rain gutters and that helps.
Stone fruit is more forgiving of clouds and rain but have their own issues around water. Light, and probably heat, is only 53% inside vs outside. Thus, water use inside is about half of outside. Too much water ruins stone fruit quality. But too much water stress during summer, knocks out flowers for the next season on sweet cherries and apricot. Less so on pluots. Not an issue on peach/nectarine.
There is also an issue around pollination if the GH is too warm by day during bloom. 80s and 90s during bloom are too warm. 70s are OK, I think. So that time of year 40s at night and 70s during the day. Misting the trees during bloom might also help. High heat that time of year means low humidity during the day. Both may be an issue for pollination.
Greenhouse design is important. Most greenhouses lack sufficient heating and cooling systems. You want high light but that comes with high heat load. My GH roof is covered in two layers of Palring 175. That gives light inside just above 50% of outside. DLI, daily light integral recently, of 23 inside vs 44 outside. DLI of 23 in summer is adequate. I expect about 10 inside in winter, maybe even less and that’s in a sunny climate at 30N latitude.
The best places in the US to grow fruit in a GH would be at about 3500 to 4500 feet elevation in west Texas, NM, AZ, and nearby areas. I’m at 4500 ft in SW Texas. Dew points below 60 in summer and below 30 in winter are good. The lower the better.
Achieving chilling in the GH is easy in my climate. I can get 700 Utah hours in 45 days anytime from mid Nov thru Febr. Dec and Jan the best. This is done with a combination of shading and running the wet wall and heater as needed. Maintain 37-42 at night and hold days as cool as possible. Mostly my system maintains 37-60F with average outside at 60/30.
The best adapted fruits and also best tasting are stone fruits. I think that would be the case in colder climates like CO and in more extreme climates like zone 3-6. With long cold winters the GH could be run with a 6-7 month growing season rather than 10.5 months here. In those cold climates chilling won’t be an issue. Winter temperatures could be maintained just above damaging levels from leaf drop until bloom. That would drastically cut heating cost. Heating to zero won’t be expensive in winter even in zone 3-4.
In my setup other well adapted and great tasting fruits are citrus, grapes, figs, blueberries, persimmon, and berries.
Pest control revolves around spider mites. Right now, I’m using horticultural oil. 2x a month in summer, 1x spring and fall. But I recently had some concerns about how safe that is to eat the fruit. I’m going to look at other options like cold pressed neem oil.
The pluses of GH growing include few pest issues and no losses to critters, freezes, or hail. I also enjoy the fact that I can control temperatures and seasons to an extent impossible outside.
I always felt my fruit came up somewhat short when growing outside for 35 years before having a greenhouse. My best greenhouse fruit has been nothing short of spectacular…!!
I’ve learned a lot about fruit growing by having a greenhouse for the last 20 years. But don’t have it all figured out yet.
