The Cheapest Greenhouse Available

Grow Plants using Milk Jug Greenhouses

I’ve got a greenhouse half-built, frozen garden beds, and I am itching to start growing some food. I need to be resourceful. There is a foot of snow on the ground and March is right around the corner. I have started perennial flowers in milk jugs before, but I asked myself:

Can I start vegetable seeds in a milk jug?

This isn’t winter sowing for cold stratification. This is using a plastic jug as a mini greenhouse because temperatures in February are decent, I'm seeing lots of sunshine, and I want to get seeds started as cheap as possible.

Turns out, Yes, you can start vegetable seeds in a milk jug. The best option is a one-gallon jug made with semi-transparent, food-grade plastic. The plastic from a milk jug acts as a miniature greenhouse and the seeds can germinate weeks ahead of direct seeding. For best results, use 2-3 inches of quality seed starting mix to make sure seeds have the nutrients they need.

Let’s explore how to get an early start on vegetables using a milk jug.

Again, this is not for cold stratification. If these vegetables get too cold they will need to be brought inside to protect them from temperatures below freezing. Putting them in a milk jug acts as a mini-greenhouse and can give you a few weeks head start for seedlings. 

Many of the seeds I’ve started with are vegetables like arugula, broccoli, kale, swiss chard which can withstand colder temperatures. These are good options for this technique from my personal experience.

This method is cheap, but can be less reliable than just using a grow tower or greenhouse.

How to Start Vegetable Seeds in a Milk Jug:

Let's dive into these steps a little deeper:

1. Make sure the plastic is food grade plastic so it won’t break down and leach into soil (don’t us an old cleaner bottle or something)

2. Clean the jug out with soap and water or 10% bleach solution (other 90% water) but I have used soap and it works well enough. Dairy leftovers can stink bad and lead to mold.

3. Take cap off for ventilation and cut in half but leave the handle in tact as a hinge at the bottom.

4. Mix the seed starting mix by hand to make it moist BEFORE putting it in the jug. This is important to retain moisture and help seeds stick in one place when put in the soil

5. Fill the jug with seed starting mix 2-3 inches tall and set the seeds into the soil at their preferred depth.

6. Label and date the seeds and when you put them in there.

7. Tape it off and let the seeds do their thing.

Using Milk Jugs as “Mini-Greenhouses”

These milk jugs act like mini-greenhouses. Light from the sun enters through the plastic. Either translucent or the opaque white plastic lets some of the sun's energy in. The plants and soil absorb the light converting it into heat energy (I know, pretty scientific). 

This is part of what makes the inside of the milk jug plastic wet from condensation. 

The soil, seeds, and plants retain heat. The heat is also trapped by the walls and has difficulty escaping the milk jug leading to increasing the temperature on the inside of the jug leading to germination and sprouting.

This method will require you to pay attention to what is going on inside of the jugs as well as what is happening with the weather to make sure the temperature isn’t dropping below freezing.

It’s is worth mentioning that I am in zone 6. We get mild winters and maybe a foot or two of snow and average winter temperatures 4-8 weeks before spring around 30 degrees Fahrenheit, I have to bring them inside for days at a time and put the jugs near a window sill.

This milk jug method is cheap but requires a little more attention than grow lights or greenhouse.

Clear vs. White Milk Jug?

Personally, I have only used the milk jugs with translucent plastic, not clear. From my experience, the clear stuff can lead to damage of seedlings on a warm day by letting in too much light and basically magnifying the sun's rays.

Where I live, it can stay relatively warm and sunny throughout the year, touching 50 and 60 degrees Fahrenheit on random days even in the winter. For example, it’s early February at the time I’m writing this and it was 54 degrees today. That sunshine would scorch those seedlings and kill them if I used the clear jugs.

The translucent, opaque color of the classic milk or water jug works well to trap the sun’s rays but not overexpose them.

What kind of dirt should I use in milk jugs?

 We have seen success with seed starting mix. If you are serious  about getting these plants growing, then use the money you save avoiding a greenhouse or or cold frame towards a nice seed starting mix. 

Some people say they want a soilless mix because it's mostly amended with perlite Sphagnum Moss or vermiculite. Ideally for winter sowing is a four-part compost: 2 parts perlite and two parts peat moss. this gives you the nutrients necessary to get seeds started off on the right root.

Will milk jug protect plants from frost?

 The milk-jug itself traps Heat but the soil and plant material is what holds heat. A few inches of soggy dirt and soil will not protect the plants from frost.

When Should I Start the Seeds?

Use the information on the seed packets. The stuff is extremely valuable and simplifies the  process. are some things that you'll see on the back of seed packets:

1. ‘ Direct sow after all danger of Frost has passed’ it means you can plant it late winter or early spring and put it straight into the ground.  The guessing game is whether or not the last frost has passed…farmers almanac helps with that estimation. This Doesn't necessarily mean that you need to have cold stratification, but you can utilize milk jugs under grow lights, or outside if it's warm enough to get seeds started early. You can jam a bunch of seeds really close to gather so they can Sprout and then move them into individual larger containers so they can grow more.

2. ‘Start indoors 3-4 weeks before average last frost’ start in milk jug late winter

3. ‘Start 4-6 weeks before average last frost’ sow seeds in your jugs early to mid winter for best and fastest results.

I’ll give an example. I used this milk jug method for my broccoli, and I had to take it in and out a few times to put it in front of the window sill on colder days. It germinated quickly and within 3 weeks it already had it's true leaves.

How many seeds can I put in the milk jug?

You can crowd the seeds in here as they won't get a deep root system and fight for nutrients.

When do I take seeds out of milk jugs?

You can open up the milk jugs and let the seedlings harden off by getting a taste of the elements. When the time is right and you see 2 sets of true leaves, you can replant these by getting a spoon and carefully scooping the root system into a new spot with plenty of room.

Does the milk jug break down?

 If this is food grade plastic it shouldn't break down at least in the timeline of when you are using them. It's plastic come on it won't ever break down. 

Does Fungus form in Milk Jugs?

If the jug is cleaned out of any dairy residue that should help with avoiding any sort of mold or fungus. You should also make sure that you put 5 to 10 holes in the bottom of the milk jug for drainage.

Otherwise, fungus and bold could form as the water sits in the dirt and accumulates on the plastic. This is also important because you don't want your roots to have rot.

Also, leave the cap off for ventilation.

Conclusion:

Buy quality seed starting mix. Mix it by hand and get it moist before putting it in the jug. That was all the difference for me.

Benefits of Milk Jugs for Sowing Seeds

1. Faster start than putting seeds directly in the ground

2. No need to build expensive lighting setup

3. Free

4. The height you need for sprouting seeds

5. Easy Ventilation and drainage

6. Translucent plastic > clear

Drawbacks:

1. Less reliable

2. more time and attention to weather