To take up space at the bottom of a deep empty bed, you could choose to add no more than 50 centimetres of small branches, leaves, mulch, pine needles, or other woody organic matter, and then add the other recommended raised bed soil and compost on top. The woody debris eventually breaks down and feed the ground as a carbon source over time. However, WE do not recommend adding non-organic matter such as rocks, plastic bottles or other random materials to take up space in your bed.
Try to fill your beds up to the top! When you first water your raised bed garden, it will compact and sink a little deeper. Depending on how much it sinks, you may want to top it off with another layer of compost.
Mulching. Materials such as compost, leaves, straw, or pine needles can be used to top off a bed and increase moisture retention. Use what is most readily available and appealing to you!
Approach making a raised bed garden as if you were making the most comfortable bed in which our seeds, seedlings and plants will sleep, rest, feed and grow strong and healthy!
Build, scavenge, or buy a container in which to house a multi-layered ‘mattress’ of organic filling materials…
The surface on which you build your garden is either floor, paving or ground cover. If it is bare earth or lawn begin with layering the first layer with old cardboard, thick paper, or any bio-degradable non-toxic material. In the case of containers without bases, lay this first layer slightly broader and longer than the pre-built box you plan to use. Place the container atop this layer to hold it in place. The importance of this layer is to kill any pre-existing weeds or prevent unwanted growth rising through the bed.
Place a layer of more significant, older dead, dried and porous logs or branches in the base of the garden bed. As these logs and components are added, infill them with any top or garden soil (but not expensive compost) and compact.
This base log layer will act as a sponge to retain moisture and host its own variety of beneficial microbe species and plant matter nutrients as it is. This layer will host many beneficial bugs and microbes as it naturally breaks down, aerates, and feeds the roots of the plants.
This next step will be to add a layer of a combination of smaller, thinner branches, sticks, leaf litter, bark chips and clean organic matter. This bed will biodegrade quicker than the base layer and make for a wonderfully aerated medium for roots to search through and grow through whilst hosting separate but equally beneficial species of worms and beneficial microbes. Make this layer deeper than one third, as it will compact as the next layers get added above. It is important to note that each layer is beneficial to each other in terms of creating a perfect 'biome or ecosystem' for your plants.
The following layer will be of finer leaf matter, wood chips, plant residues, manure, and any other compostable plant-based bio-degradable material.
The next layer will be your seed or seedling bedding of rich organic compost. The thickness of compost or peat in a reasonably large raised bed of about 800mm or 30 inches needs only to be as thick as the root requirements of the vegetables or flowers planted.
40 centimeters is a good starting depth for this layer. With subsidence, this layer will need to be topped up over time.
PLANT YOUR SEEDS, SEEDLINGS OR SHRUBS!
Finally, once you have planted your seeds or seedlings, cover with a generous, protective layer of straw or dried cut grass as a MULCH. This layer will act as a protective top for sprouting seedlings and roots whilst they are vulnerable to shifts in temperature, direct sunlight, and dehydration. This 'mulch duvet' will also be a barrier for the whole ecosystem, against moisture loss, temperature loss, leaching, erosion and drop-pitting during rain or watering.
This method can be constructed in the spring, and your raised bed garden can be planted right away, or it can be started in autumn to allow the organic matter to break down all winter long and then the raised bed garden garden can be planted the following spring. The buried debris will feed the soil for about five years.
Pro-tip, the depths of all these layers may vary depending on what crop you plan to cultivate. All the layers should be in proportion to the rest of the raised bed garden.