Mini-Greenhouse Garden Hack

By Sage OsterfeldPublished On: January 26th, 20234.1 min readCategories: Garden

Get an early start on your growing season with a super simple high tunnel hack

This mini greenhouse can warm the soil by 20 degrees or more

I live in an area where there are a lot of outdoor nurseries and plant growing operations. Even though it’s in USDA Zone 9, it’s hilly country, so while the days might get into the 60’s, the valleys create microclimates that will drop near or below freezing overnight from mid-January to mid-March, which is a problem if you’re growing subtropicals, succulents, and other frost-tender plants.

Normally you could protect the plants with floating row cover or plastic sheet tunnels, but mid-winter we also get gusty Santa Ana winds which shred those lightweight cold protections and turn them into kites.

The little cut flower farm in my neighborhood has a great way to solve the problem using corrugated polycarbonate roofing panels and wire. The panels, which come in 26-inch by 6-foot lengths from the local home improvement store, are cut into 2-foot segments and secured across the middle with a piece of stiff wire, turning them into mini hoop houses.

Then they’re placed over the rows with a gap of about a foot between them to ensure air circulation and heat dispersion . A fabric pin over the wire holds them in place in the wind.

And viola! Awesome little greenhouses that warm the soil and let them start plants in-ground a full six weeks before spring.

They can also be adjusted for height, so the plants have a little extra shelter as they grow. And when these little greenhouses are no longer necessary, the wire unhooks and the panels can be stacked and stored flat until the fall.

Oh, and unlike plastic and row cover fabric, they don’t tear or shred and will last 10 years or more.

Pretty cool.

I used these polycarbonate panels on my greenhouse a few years ago and had a couple extra stored behind the shed, so I decided to make a couple of these and test them in the garden myself. I wasn’t disappointed, and I don’t think you will be either.

Here’s how to make them:

Supplies:

Piece of clear polycarbonate panel
Coat hanger

Tools:

Tin snips or a sharp box knife
Wire cutters
Drill or a hammer and large nail

Instructions:

1) Using your tin snips or box knife, cut the polycarbonate panel into 2-foot lengths.

Cut the panel into segments

2) Drill (or punch with a nail) a whole in the top, center and bottom edges of each panel.

cut polycarbonate panel with diagram showing where to drill holes

Drill (or punch) holes in the panel in the following locations

3) Cut the coat hanger as shown. This should give you a wire about 18-inches long with a hook at either end.

Cut the coat hanger with wire cutters as shown

4) Find the center of your wire, and bend it about 6-inches on either side. This will allow you to make the hoop taller or wider based on your needs.

Bend the wire upward 6-inched from the center

5) Curve the panel and pass the wire through the holes in the center on either side.

Pass the wire through either side to create a half-dome

The mini greenhouse assembled

The extra holes at the ends of the panel can be used to secure it to the ground with fabric pins, or to join two panels together to make a longer tunnel.

The mini greenhouse in a garden bed.

Note these panels absorb a lot of heat, so if you join multiple panels together, the air circulation will be reduced and the middle will be much warmer (20° or more) than the edges.

That’s it!

As you can see from my own tests, these little greenhouses are very good at capturing heat, and will add 10 or more degrees to your soil in a matter of minutes.

Temperature before the greenhouse

warmer soil temperature after greenhouse

The temperature 10 minutes after the greenhouse is in place

I tested one all day, and it averaged about 25 degrees warmer than the air, and trapped enough heat in the soil so that it never fell below 40 degrees at night, even when the surrounding area fell to 32 degrees.

My garden is on a west-facing slope, so it’s shaded for several hours longer in the morning than the nursery, so I still need to wait a couple more weeks before I put anything into the ground.

But I am looking forward to these little greenhouses helping me get started outdoors four to six weeks than I would be able to otherwise!

Speak up!

Have you built a similar mini-greenhouse for your garden? Tell me about it in the comments below so I can share it with others!

About the Author

Sage Osterfeld

I’m just a guy with nearly an acre of dirt, a nice little mid-century ranch house and a near-perfect climate. But in my mind I’m a landscaper survivalist craftsman chef naturalist with a barbeque the size of a VW and my own cable TV show. I like to write about the stuff I build, grow and see here at Sage's Acre.

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Mini-Greenhouse Garden Hack