Raised Beds: A Great Option for Not-So-Great Soil
Are you looking to start your plantings earlier and possibly stretch your harvest date further? Look no more than raised beds. Raised beds are nothing more than elevating soil or mix, which allows the soil to warm faster in the spring and to cool slower in the autumn months. In some regions, depending on climate, using raised beds lengthens the growing time by an extra month on either end of the regular growing cycle. That can be a real advantage in getting strong, healthy plants and trees.
Arbor Day Farm is currently experimenting with our own raised beds. Our soil here is, for the most part, clay. This hinders good root development for young seedlings and it also eliminates the possibility of lifting (that’s “greenhouse speak” for harvesting) the seedlings at maturity. The clay would tear the lateral roots, all but killing the seedlings once they went to outplanting. With the challenge of clay soil in our area, we decided to go with raised beds as a trial when it comes to growing bareroot seedlings here at Arbor Day Farm.
We started by choosing level ground near our current growing house operations. We then staked out two sections, each 4’ by 30’. We chose 4’ because it was wide enough to still access the middle of the bed from either side. We then removed all the sod out of the area and trenched a 6” wide by 6” deep area down the middle of the bed. Next we constructed the bed walls. We chose treated lumber (2” x 12”). We lined the bed and the wood walls with a 6-millimeter thick black plastic sheeting, which will block any leaching chemicals found in the treated lumber.
Once the walls were up and the plastic down, we then laid 4” PVC drain pipe down the trench, and placed about 2” of river rock on top of the pipe as our bottom aggregate. With the river rock in place, next comes the irrigation system. We put together a simple system utilizing a typical lawn irrigation computer, ¾” PVC pipe, and a major brand riser/irrigation head. Next layer: soil.
Our soil mix included yard compost, peat, sand, and vermiculite. Once mixed, our recipe gave us the consistency we were looking for. The yard compost was an easy choice because its readily available in our area and it is cheap. (Hey, we all have a budget, right?) The peat is another natural organic that gives good porosity. The sand gives the mix its compaction. Without sand, the mix would be fluffy. And, last but not least, the vermiculite (the round white balls you always see in purchased container plants) gives the mix great porosity and good drainage characteristics. We went heavier on the yard compost, which caused our pH to start high. However, the pH will be adjusted during the growth of the seedlings with an acid injection at each watering.
Once our soil recipe was fine-tuned, we filled the beds all the way to the top because we knew the soil mix would settle. And we were right. We finished the beds in late May and the soil has settled about two inches since then.
In our raised beds at Arbor Day Farm, we are currently trialing two shrubs and one tree species. After sowing, we covered the entire raised bed with a shadecloth and elevated it on 5′ risers, allowing the irrigation to function correctly. Once the shadecloth has served its purpose, we will convert it to a wildlife screen to ward off those pesky mammals, both large and small. This screen will stay on until lifting (harvesting) in the fall. We look to lift our mature seedlings with nothing more than a pitchfork. The tines on the fork should lift the seedlings uniformly and we can then pull them out in groups and shake off the soil mix from the roots.
This particular raised bed system is a good one to try. We’re learning the art of bareroot growing, and this system allows us to show what we’re learning to the many visitors that come through the Tree Adventure and the Lied Greenhouse here at Arbor Day Farm. This outreach is a direct connection to our mission: Inspiring People to Plant, Nurture, and Celebrate Trees. We hope you’ll come visit this summer and see for yourself.
Until next time,
Adam Howard is the Manager of Nursery Operations at Arbor Day Farm and a Certified Forester with the Society of American Foresters. He has been in the forest industry for 11 years and contributes regularly to this blog. For more information on how you can receive trees from the Arbor Day Foundation, visit arborday.org.