Mittleider grow box maintenance needs to be completed once or twice a year. Grow boxes in a Mittleider garden are filled with sand and sawdust. As the gardening season progresses the sawdust will decompose. As it decomposes the growing medium in the grow box drops. The lost material will need to be replaced.
How often to add more sawdust
Here on our little homestead we will do Mittleider grow box maintenance at before each gardening season right before planting. In grow boxes where we rotate in a new crop for our fall gardens the grow box will get topped off a second time. There are a couple things to consider when deciding how often to add more sawdust.
What to consider when deciding when top off a grow box
Performing mid season Mittleider grow box maintenance on crops like kale or tomatoes would be a problem because the fruit and growing tip wouldn’t get buried with the added sawdust. To try and top off a bed with crops such as cabbage or spinach you would end up partially burying your crop.
A common side effect of adding fresh sawdust is a drop in available nitrogen. Watch for nitrogen deficiencies if topping of the grow box during the growing season. If you see symptoms of a lack of nitrogen you will need to correct for nitrogen deficiency. Urea is a common nitrogen source that is widely available.
What sawdust will work
A common question I see asked is what type of sawdust is correct to use and what size it should be. The type is easy, anything but walnut sawdust will work. Walnut contains tannin that will kill seedlings and severally stunt the growth of established plants. Avoid any sawdust that could possibly contain walnut.
The size of the sawdust is important. Anything too large, or too small, can cause serious problems in your Mittleider grow box. See my blog entry on identifying the correct sawdust size. Another acceptable source of sawdust that can be bought is from equine pellets. Equine pellets cost around $6 per 40 pound bag and will roughly triple in volume once wet,
Mittleider grow box maintenance in our potato box
In this video you will see our potato and sweet potato grow box. Because of the length of the sweet potato growing season this box is only filled once a year. It gets topped off right before we transplant the sweet potatoes.
Why I don’t like a 4 foot grow boxes in a Mittleider garden
I’ll be honest here, I don’t like 4 foot wide grow boxes. They’re popular and you will see all kinds of articles and videos where they are recommended. They require too much bending over of the gardener for my liking. For a Mittleider garden I think they’re generally a mistake.
It is difficult to tend to stuff in it and you can’t reach from one side to the other. This becomes particularly evident when things are grown vertically. Reaching towards the middle of the box to prune or harvest from a mature plant is near impossible. Sure, you can push through and get it done but you risk damaging plants. you could even knock off nearly ripe vegetables.
The top of these tomatoes in the picture below are a little more than 3 feet apart. I can only prune and harvest easily by going down each side of my grow box. Growing these tomatoes in a 4 foot wide grow box would prevent the easy access necessary to properly care for our plants.
Are there good uses for 4 foot grow boxes?
There are indeed. First, if you’re on a very tight budget yet determined to have a grow box the wide grow boxes will work. For the cost of two 18 inch wide boxes you can build one 4 foot box. But if you’re on a budget I’d highly recommend growing in your own soil.
The only time I might recommend a 4 foot wide grow box in a Mittleider garden is for growing potatoes. We have heavy clay soil here in Missouri. Our grow box dedicated to growing potatoes allows them to sprawl and makes harvest a breeze. It is isolated from the rest of the garden and is used only for potatoes. In this picture below we grew sweet potatoes last year.
What size grow box do I prefer?
That’s easy. An 18 inch wide grow box is ideal, particularity for vertical crops in my Mittleider garden. We built these grow boxes in our in the garden green house.
When it comes to gardening in grow boxes in your Mittleider garden it is extremely important to choose and obtain the correct sawdust. Not just any sawdust will work as your Mittleider garden sawdust.
Most any type of tree will work, with the exception of black walnut. Black walnut has toxins in it called juglone that can stunt, deform or even kill other plants. If the sawmill cuts black walnut you should avoid that sawdust, even if it is free. Do nut use walnut for your Mittleider garden sawdust for any reason.
The only kind of sawmill that you should consider as a good source of sawdust is one that runs a large diameter circular saw blade. The blade kerf is wide enough that you will get the correct size of sawdust particles. See the video at the bottom of this post to see the correct size and an example of a sawdust that is far too fine.
Band saw sawmills
Sawmills that run band saw sawmills are becoming popular and can be found on homesteads where folks make their own lumber. Unfortunately the kerf is much smaller on these types of mills and the sawdust particles are too fine for our purposes and isn’t a good source for our Mittleider garden sawdust.
The size of the sawdust particles is important
Have sawdust that is too large, such as wood chips, will result in poor drainage and souring of your growing medium. This can prevent water from getting to our plant roots and even cut off oxygen. Either can surely kill your plants. (Remember the 6 laws of plant growth?) Planers or even wood chippers would be a common source of wood chips that are too large.
Just as too large can kill your plants, so can sawdust particles which are too fine. This sawdust is would be collected from table saws, band saws, and that cool saw you see in stores like Home Depot and Lowe’s. This sawdust is far too fine and can cause compaction. Once hard and compacted your plants can struggle to get water and nutrients. This fine sawdust is often free, but it can cause real problems in your garden and needs to be avoided.
What if there are no sawmills near me?
If you’re unable to locate sawmills near you and are determined to grow in sand and sawdust then you can buy equine pellets. When the pellets become wet they turn into a usable pine sawdust that works well. It takes just a few minutes to transform the pellets into a usable sawdust.
After receiving a good hard frost we decided to dig up our Georgia Jet sweet potatoes. We are tickled with our harvest that ended up filling a wheel barrow. Not sure what the total weight as we just didn’t weigh them.
They were growing in a 4 foot wide and 15 foot long grow box that was filled with a mixture of sand and sawdust. Digging them was very easy, we used only our hands to sweep the sand and sawdust out of the way to expose the sweet potatoes.
You can see the outline of the growbox in the picture below. As you can see, they grew like mad. Next year I will prune the vines as they grow beyond the box.
This was the average size of what we harvested. They grew easily in our sand and sawdust and weren’t mishappen.
This is the completed harvest here in the wheel barrow. Not a bad haul for a 4 x 15 foot growing area!
We now have 32 Georgia Jets sweet potato slips in our 4′ X 15′ grow box. I’ve never knowingly eaten one before but wanted to give them a try. This is also our first year growing any sweet potato in a grow box with a sand and sawdust growing medium. Last year digging potatoes in the heavy clay soil was a lot like work. This year it should be so easy.
I’m still working on clearing the grass and weeds around the grow box. When finished there will be a bare dirt border 5 feet from the box. By doing this it helps keep insects away from our vegetables. It also helps keep weeds from getting into the grow box and competitimg with the vegetables for the water and fertilizers as well as space and light.
With the grow boxes filled with medium and seedlings transplanted our next project was the completion of the automated watering system. Lesson 16 in the MGC book, page 125, covers the construction details of the automated watering system. Three holes were drilled every 4 inches of the 3/4 inch schedule 30 pvc pipe to water our grow box. The holes are small, we ordered the number 57 drill bit from Amazon as our local stores don’t carry anything so small.
The holes are positioned to get a stream of water to squirt straight down into the bed and the other two at an angle towards the outside of the box. Our system will be drained in the winter as it turns cold. For all the piping that is below ground we elected to go with schedule 40 to help ensure we don’t have to dig up a broken pipe.
Currently we water by connecting a garden hose to the system. At some point we may go high tech and use a truly automated system.
Our grow boxes and in garden green house is beginning to come together. We now have two 18 inch wide by 30 feet long grow boxes built from treated 2 x 8 lumber in our garden. To those grow boxes we are adding the structure to allow us to grow vertically and support our in garden green house. The grow box construction is covered on page 78 of the MGC.
Before mixing and adding our growing medium, sawdust and sand for us, we added our pre-plant mix to the bottom of the grow box. Page 85 in the MGC book tells us to add one ounce per linear foot. So to this 30 foot grow box we measured out and added 30 ounces of pre-plant fertilizer. The grow box is now ready to be filled with a custom growing medium.
We utilized the free sawdust we were able to get locally to make our custom growing medium and sand. Following the MGC book, we mixed three parts sawdust to one part sand. This is mixed by volume and NOT by weight.
One 50 pound bag of all purpose sand filled a 5 gallon bucket so we used that as our measuring tool. After each bucketful of sawdust was added to our wheel barrow we added approximately 1/3 of a bag of sand. After the 15 gallons of saw dust and 5 gallons of sand were in the wheel barrow we mixed everything with a shovel before pouring it into the grow box. Each 10 foot section of our 30 foot grow boxes required 15 gallons of sand and 135 gallons of sand.
Once the grow box is full and level we have to add our pre-plant and weekly fertilizer to the medium and mix it in thoroughly. Page 87 in the MGC book calls for 1 ounce per linear foot of the pre-plant fertilizer mix and 1/2 ounce per linear foot of the weekly feed.
The fertilizer can be mixed in with a potatoe for or a shovel, but we have an attachment for our gas powered weed eater and will be using it.
These are a little late in coming, but both boxes are now full and there is more progress on the green house portion. Later today we will order the green house plastic to cover the structure
These 30 foot long and 18 inch wide boxes are being built for our fall crops. We will also be building an in garden green house over these boxes. This will allow us to extend our growing season by 12 weeks.
Once construction is complete they will be filled with a mix of sand and sawdust as our growing medium. As that growing medium contains no nutrients for the plants we will be providing all of the necessary nutrients via an inexpensive mix that is applied on a weekly basis.
The PVC you see next to the treated 2×8 serve a few purposes. They help hold the boxes level and in place once filled with our sawdust and sand growing medium. They also support the a-frames we will be making by heating and bending some half inch pvc. Once the mini a-frames are in place they can then be covered with green house plastic to protect our fall crops.
Next to the grow boxes you can see our sweet potatoes growing like mad.
Our quest to be self reliant and grow a healthy garden