Start Seeds in a Mini Greenhouse for Success

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start seeds in a mini greenhouse

While I love growing plants and flowers from seeds, I’ve not been very successful. They sprout well when I have them inside, but when it’s time to take them outside, the hardening process is rarely successful for me. This year, I thought I’d try to start seeds in a mini greenhouse.

I’m trying out a pair of 4 tier portable mini greenhouses this Spring, and so far I’m seeing excellent results! Additionally, this experiment is doubly interesting because 90% of the seeds I’m starting are harvested from last Fall’s plants.

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What I’m Growing From Seeds 2023

My Location: USDA Zone 8

Santa delivered these two greenhouses from my amazon wish list. Early March 2023, I assembled them and brought them outside and outfitted them to grow my seeds. I cannot wait to start seeds in a mini greenhouse! Last Fall, I collected (harvested) seeds from these plants, already established in my gardens:

The only commercially-packaged seeds I’m growing are Mexican Sunflowers. January and February couldn’t pass fast enough, I was raring to start seeds in a mini greenhouses!

crocosmia seeds i think

I’ve read that Crocosmia are difficult to grow from seed. I’m going to give it my best, though, since I adore Crocosmia “Lucifer” plants and flowers for their sweeping elegant look.

In the image, I’m not sure what are Crocosmia seeds and what are chaff. I’ll plant it all.

Here’s a gallery of seeds I’m starting in my greenhouse:

Mini Greenhouse I’m Using

start seeds in a mini greenhouse like this one

I was gifted a pair of Amazon mini greenhouses, but you can buy mini greenhouses in many places. You’ve probably seen mini greenhouses like this at larger retailers. It’s a 4 tier mini greenhouse. They’re essentially a kit of very lightweight shelves with a zippered clear plastic container enclosure, supported by thin metal scaffolding for a frame. They come in various sizes from one to two shelves (2 cubic ft), to walk-in size with three banks of shelves (100 square feet.)

They rely on lightweight metal tubing and plastic braces for the framing. Lightweight wire grids are shelves that snap onto the levels. And a form-fitting, zipper-access plastic enclosure shelters the occupants. Ideally, you will place this greenhouse where it receives as much sun as possible. Now, if the sun is out the inside of the greenhouse will get quite warm – on a sunny 55° F degree day, my thermometer registered over 100° F inside the greenhouse! Also, the humidity level remains very high.

Do You Need a Mini Greenhouse Heater?

To start seeds in a mini greenhouse, you might wonder: do you need a mini greenhouse heater? It depends upon where you live and the weather you get in March and April. In the early Spring, if the temperature drops to or below frost (35° F) you have 2 options:

  • Wait until there’s no more chance of frost before beginning seeds in your mini greenhouse, or
  • Install a few warming lights inside of the mini greenhouse.

Remember, only halogen or incandescent bulbs give off much heat. CFL (compact fluorescent light) bulbs put off hardly any heat, and LED bulbs put out close to zero heat. It would serve no purpose to use these last two types of bulbs to heat your greenhouse.

I put in a simple desk lamp in one greenhouse and left it on continually. Monitoring my remote thermometer device, I can see that the lamp keeps the inside of the mini greenhouse above the frost level all day when the sun is shining. But night time if the temperature drops to or below the frost level, one lamp isn’t going to do. I graduated to a clamp lamp with a heating halogen light bulb and set it on a timer, so it’s on in the dark hours. If it stays cold during the daytime, I just leave it on continually.

Start Seeds in a Mini Greenhouse
Small desk lamp using an incandescent bulb placed at the bottom shelf provides a modicum of heat

Opening the zippered closure is a neat sensation. You feel the warm, moist air and you also take in the aroma of soil and growing things!

Use a specially designed seed starting soil for your seedlings
Use a specially designed seed starting soil
for your seedlings

Materials I Use Starting Seeds in a Mini Greenhouse

Now, as I said the shelves in the mini greenhouse aren’t very heavy duty, I needed to use trays to place my seedling cups on. The lighter-weight the better. Plastic cafeteria trays are a great option, metal cookie sheets, rectangular cake pans. The important thing is that you should use trays that have some depth to them, at least 1/2″ – this way, you can water a whole tray of seedling cups at the bottom so you won’t disrupt the growing seeds.

Also, I saved a number of fiber egg cartons. I made some strategic cuts so that when it’s time to separate the seedlings and put them into a larger container individually, the individual compartments will separate easily.

Watering Your Seedlings

You can use a mister to water your seedling cups, but I found that watering the tray that the cups sit in provides the seed cups with plenty of water.

Start Seeds in a Mini Greenhouse
Add water to the seedling cups’ tray, not the seedling cups themselves so you won’t disturb the seeds.
use grass clippings to brew a nitrogen rich water "tea"

A note on water use: I have started to retain garden, grass, and weed clippings to use as fertilizer. Place a few handfuls of these waste trimmings into a plastic bucket, fill with water (and leave outside uncovered so you’ll collect rain water) and the water will become infused with the nutrients!

Starting Seeds In a Mini Greenhouse Progress

First to Sprout? Mexican Sunflowers!

Start Seeds in a Mini Greenhouse

I’ll try not to get upset that a pre-packaged seed packet sprouted before any of my own hand-harvested seeds…

It’s now been about a month since the first tray of seeds have been sown. So far, Crocosmia, Coneflowers, and, surprisingly, the Shasta Daisies have yet to sprout. However, all the other seeds are growing strong! Snap dragons, Milkweed, Cardinal climbers, Rudbeckias, Hollyhocks, Salvias are all looking terrific one month in.

Lessons Learned – Cold Stratification

Shasta daisies and Coneflowers didn’t germinate at all. It is a surprise, considering how hardy these plants are. I have since learned that wildflowers and most perennial seeds benefit from cold stratification. Cold stratification is a gardening technique that involves exposing certain seeds to a period of cold and often moist conditions to simulate winter. Essentially cold stratification mimics the natural conditions seeds would go through in the wild to trigger their growth when the weather warms up.

How can you mimic a period of winter? Two ways: Refrigerate your seeds in an airtight container for at least 30 days before sowing. I plan on giving my 2024 seeds a refrigerator period of at least 60 days starting in December. I will take them out and sow them in my indoor starter cups and process February 1st.

Lessons Learned – Fertilizer

I provided zero fertilizer to my sprouts. I have learned that seeds contain all they need to germinate and grow their first set of leaves called cotyledons. After this point, seedlings can benefit from light fertilizer. While using grass clippings in water to make a fertilizer tea can be effective, it might introduce unwanted weed spores or seeds where they’re not wanted. Slow release fertilizer like Osmocote is a good choice for young plants.

Lessons Learned – Seedling Cups

While my attempt to use egg fiber carton cells for seedlings was noble, they’re really not suitable for the task. I noted all of the fiber egg carton cups got moldy and green, and the germination rate was quite low, compared to the peat cups I switched to.

Lessons Learned – Climate Control

I’ve investigated reusing clear plastic tubs that spinach leaves come in to use as humidity domes. These are quite effective in maintaining moisture and a little warmth.

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After years of denying it, Donald finally admits one passion in life is gardening. More specifically: growing seeds, plants, flowers and edibles and helping them to be the best possible. Neighbors call him a Green Thumb. He lives in Western Washington with his wife of 24+ years and three cats.

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