Intro: Greenhouse Modifications for Hot Climates
In the late summer of 2015 I was given a 10 x 12' (3 x 3.7 m) greenhouse as an early holiday present. It was on sale so my family decided it was a good time to purchase one.
Currently, I am living in Bullhead City, Arizona. The summer temperatures compete with Death Valley and in the winter it can get below freezing. The temperature extremes along with the strong winds in Bullhead City each create their own set of challenges that required modification to the greenhouse.
With the right modifications the greenhouse became the perfect climate for growing rare tropical plants, starting seedlings, and keep my plants alive all year. This instructable is not a step-by-step of each sub project, but rather an overview to give you guidance if you are unsure where to start.
Also, I'm working on a website about the rest of my gardening trials and errors called pugsinthegarden.com and hopefully the site should be ready in a few months.
Step 1: Build Greenhouse +/- Foundation
Obviously, there are many creative people that can build their own greenhouse from scratch. I am not one of those people.
In the pictures you can see that I have laid the base of the greenhouse over gravel. Later, I realized that this was a bad idea. If you can, the better option is to place about 1.5 feet (~ 46 cm) deep foundation with about 4'' (~10 cm) sitting above ground level. Why is this? To keep it from blowing away? No! The problem with keeping your foundation at ground level is when you water your plants the water will flow out rather than down. I can't tell you enough how much I deeply regret not making an underground foundation. Alternatively, if you are dead set against this or you have no idea how to make a cement foundation you can make a small garden around the perimeter of your greenhouse. This way when the water flows outwards it also waters the plants outside the greenhouse. Also, if you are going to have a tree next to you greenhouse (preferably on the north side so that it doesn't block light) allowing the water to flow outward would allow you to water the tree and the greenhouse simultaneously.
The point is, you want to decide whether or not you want a foundation based on how best to use your water. If you decide to simultaneously water your tree or a surrounding garden then the water isn't wasted.
Step 2: Cooling System
If you have a greenhouse without a cooling system, it's just going to get hotter inside the greenhouse. Evaporation coolers are pretty much the way to go in terms of energy usage and maintenance. Also remember that your plants are not going to necessarily be comfortable at temperatures you're happy in. Really, most of you plants are probably going to be happiest at about 90 F (~ 32 C).
In the picture you can see I've constructed a wooden structure to hold up my evaporative cooler. I also wanted a small table or bench, so made it chair shaped on the inside of the greenhouse.
The evaporative cooler I purchased can be used as both an evaporation cooler or just a fan. This is good for the fall season when you don't want excessive moisture in the greenhouse, but it gets a little too warm during the days.
One thing to note is that if you have very hard water you will likely end up buying a new pump every year ($14-35 ea). You can find an assay of your ground water online or from your water company. A 2009 assay showed that we have 51-204 ppm calcium (very hard), sodium 98-180 ppm (hard), and sulfate 101.6-430.6 ppm (very hard). These are all naturally occurring the from erosion of volcanic rocks around our basin (fun facts!).
Lastly, during the first summer I believed that I should leave the roof panels open to let the hot air escape. After sealing the windows, I can tell you that there is still enough holes to let hot air escape. A sealed greenhouse is better.
Step 3: Sealing Cracks
With my greenhouse I was never able to keep the panels from blowing off in wind storms no matter how many clips I placed on them (and I bought extra!). Every time a panel blew off it might get bent and scratched up (bad!). Retrospectively, I also realize that a lot of air was escaping prior to sealing the greenhouse. To solve this problem I sealed each panel with silicone. Sure, this will be more of a pain to deal with if I need to replaced a panel, but so far it has worked out great! This also keeps the entire structure from shifting in strong winds.
Step 4: Shade Cloth
During the first summer when the plants were just getting started I needed shade cloth. There was no ecosystem or microclimate. Just a big plastic house full of dirt and tiny tiny plants under a blazing hot sun. So, I put a large shade cloth over the entire greenhouse. Given the very high temperatures I decided to go with Aluminum shade cloth. This way most of the light/heat is reflected rather than absorbed. If you would like one that is more easy to acquire and readily available I would recommend tan or green shade cloth. A black shade cloth should absorb more heat/light and that is what you are trying to avoid. Why does it matter if it's only covering the top of the greenhouse? If the shade cloth is sitting on top of the greenhouse some heat will be transferred to the greenhouse. In 120 F (~ 49 C) would you rather wear a black shirt or white shirt? If you don't know the answer to this you might not live in the desert (hint: white shirt).
Now that I have a canopy layer in the summer I am less concerned with shade cloth. This is because (1) the canopy provides dappled sunlight to the lower level plants and (2) the increased vegetation in an established ecosystem will be shedding more moisture and helping to keep the greenhouse cool (my theory).
Step 5: Planning Walkway and Laying Dirt
You can have no walkway, but eventually you'll end up stomping on plants. Also, if you let your pets in the greenhouse they will also end up trampling plants. I separated the growing area from the walkway with hollow bricks so that I could grow herbs in the bricks. Mix your native soil with good quality soil followed by a layer of composted mulch. Fill your walkway with gravel to just below the top of the brick to hold the bricks in place and to allow good drainage from the walkway. If you spill dirt the gravel filled walkway will allow the dirt to seep downward when you hose off the path.
Also, if you see mushrooms growing in your garden - don't be alarmed! Mushrooms can break down wood and are breaking down the wood in your mulch. This means they are giving you more soil! You can add more mulch from time to time when the soil level has sunk to continuously create a supply of good organic material.
Lastly, add worms! The easiest way to get them is to buy them from a fishing store. I like red worms because they are so prolific and compost quickly, but you can get normal earthworms too!
Step 6: Creating an Ecosystem
Thinks of your greenhouse like a forest. A ground cover will keep moisture in, a canopy will shade more light-sensitive plants. Will you use pesticides and kill beneficial bugs? What about predatory nematodes? What about other pest predators?
Ground Cover: This is a very important step because water loss in the desert is a big issue, but in the winter plants will still have issue with fungus such as powdery mildew. My philosophy is to plant several vigorous ground cover herbs and let them fight it out - survival of the fittest and most productive! Alternatively, if you are okay with waiting a little longer you can cultivate less aggressive, but worthwhile plants. For example, I planted sweet mint, spearmint, spicy oregano, banana mint, apple mint, pineapple mint, and several other varieties of mint. The sweet mint and oregano out compete everything, but now I'm realizing that I really like the banana and pineapple mint. I'm constantly having to fight the oregano and sweet mint for space for the banana and pineapple mint to grow. Think it over carefully. Also, one thing to note is that all the mints appear to have a much more pleasant taste in the cooler seasons and a slightly, uh, tarry (?) smell in the summer. Not everyone notices this, but it really bothers me. This might also have to do with water quality.
Low Height Plants: This is any plant that doesn't get very tall, but still holds itself above the ground cover. If you grow these make sure they aren't being completely shaded by a taller plant. These are usually small herbs (sage, electric daisy, summer savory, etc.). Strawberries also might fall into this category.
Moderate Height Plants (Light Blockers): This is anything below the canopy that still towers above the rest of the plants. Although basil doesn't get more than 2-3 feet tall I would consider this a "moderate height" plant because it also grows densely and will create a lot of shade. This is especially true for a varieties of basil such as box basil and spicy globe - they are even shorter, but very dense and block out most of the light. A vigorous pepper plant might fall into this category. Staked Geraniums can be grown upwards towards the canopy and trimming the lower leaves might make it easier for the low height plants beneath to flourish. Only place lemongrass on the north side of your greenhouse because it will grow VERY dense and tall and block out most of the light.
Canopy: These plants will provide dappled sunlight throughout the hottest part of the year. The two plants I've used successfully are vining/indeterminate tomatoes (variety: "Sweet 100") and a wild north american passionfruit (Passiflora foetida). Next year I might try a larger variety of passionfruit along with my tomatoes. In the future, when my banana trees and plumeria tree are large enough, they should create a sufficient canopy and I might not grow the tomatoes or passion fruit anymore.
Step 7: Pests
Pesticides: I don't use them. I had a lot of problems with white flies at one point in the winter, but I took out my cucumbers and grew them outside - now I have few white flies! I used to have problems with fungus gnats and since placing predatory nematodes in the greenhouse I haven't seen a single gnat. This doesn't solve all my problems though. Admittedly, I will have to spend 30 min in the summer every 1-2 days removing inchworms and caterpillars from my plants. Without pesticides I simply cannot grow (in the greenhouse) lettuce, bok choy, squash, or spinach. Despite this, there was only one day I decided to use pesticides. Earlier in the week I had become very excited about finding a praying mantis in my garden because they are ravenous predators. A plant adjacent to where the praying mantis was huddled was covered in white flies. I decided to spray the white fly infested plant very carefully to avoid any beneficial insect deaths. Unfortunately, I found my praying mantis dead the next day :( never sprayed pesticides in there again. As the greenhouse has progressed I have removed common plants with common insect problems and added rare fruit producing plants without many pest issues (ie: bananas!). This keeps my garden interesting and helps in maintaining a low pest population. And hey, if you're going to have a greenhouse why fill it with mundane stuff like lettuce when you could grow lemongrass and vanilla orchids? Also, do you want to expose your pets to that stuff?
Bees: Also note, if you spray pesticides they will likely kill bees. These little critters regularly visit my greenhouse and I love to watch them buzzing around my basil flowers. On a particularly quiet day I can hear a pleasant hum. My family, including my loving husband, used to be terrified of bees until they watched them buzzing around my greenhouse without attacking me for months (none of us have bee allergies). Sometimes they will be within inches of my face. I actually trim my different basil plants at different times so that there will hopefully always be some flowers available to them. You might say 'but what about the Africanized bees'? I have no idea how to answer that question. The University of Arizona has reported that most bees in Arizona have been Africanized already (based on genetics testing). Don't go up to a hive of bees and stick your face in it. If you have bees in your garden though, just be respectful of them and appreciate them for the valuable service they perform.
Step 8: Last Note
A greenhouse in Arizona can create a very special climate and allows you to grow plants in a greenhouse that growers in other states cannot. Be creative!
I hope that someone finds this helpful.