Filling raised garden beds: instructions & expert tips
Raised beds have been a gardening trend in the recent years. Once the garden bed is built, the filling has to be considered.
In addition to convenient and back-friendly gardening, raised beds offer a number of other advantages: they are difficult for snails and other pests to reach, they can also be used to grow vegetables in places with less fertile soil and they make it easier to keep the unwelcome weeds at bay. The heat generated by the rotting of the filling material and the lateral sunlight means that harvests can comes earlier and yields can be often more bountiful. However, a successful yield is closely linked to the material with which the raised beds are filled and therefore is solely in the gardener’s hands. This article describes everything to consider when filling and refilling raised beds and how to avoid mistakes.
An overview of filling layers
No matter whether the raised bed consists of wood, stone or any other material, the materials have to be layered. In this way, optimal growth conditions are created for the plants. The lowest layer (about 30 cm) serves as a drainage so that unnecessary water does not accumulate in the bed. Coarse materials such as branches and twigs, stones and wood chips are well suited for this purpose. The numerous pores and cavities of these materials allow the excess water to drain off better and provide oxygen for organisms living in the soil.
After the 20 to 30 cm of the draining layer, finer material such as leaves, grass cuttings, grass sods, plant-matter kitchen waste or, if necessary, topsoil is layered. If, for example, you want to fill a balcony raised bed and do not have the materials described, you can either ask a garden owner you know for help or simply purchase complete filling material for raised beds online.
The third layer is about 20 cm tall. Depending on which materials are available, leaves, crudely decomposed compost, animal bedding, grass clippings, straw or similar can be stacked on top of each other in thin layers. To quicken the decomposition process and make the raised bed more compact, mature compost can be added in between the materials.
The final layer is at least 20 cm thick and consists of a mixture of good topsoil, potting soil and mature compost. This layer of soil must never be too thin, so that the plant roots have sufficient space and nutrients at their disposal.
The recommended heights of the layers must of course be adapted to the size of the existing or planned raised bed.
Summary of the filling layers:
- 4 layers in total
- Lowest layer ~30 cm: coarse material to drain off excess water (branches, stones etc.)
- Filling layer: 20-30 cm of leaves, grass cuttings or plant kitchen waste
- Compost layer ~20 cm: mature compost, animal bedding and grass cuttings in thin layers
- Top layer ~20 cm: high-quality topsoil or store-bought plant substrate
With the lower layers, the intended use (which plants are intended to be grown in the bed) should be considered. Mediterranean herbs do not tolerate waterlogging well and should be planted in a bed that has an excellent draining system. The two lowest layers should therefore consist of coarse and poorly or not decomposable materials such as large branches, stones or gravel. With a planned and long-term planting and use of the raised bed (for example, if flowering perennials are planted), the filling material must be considered carefully as it cannot be corrected as easily as with vegetable beds. Therefore, the lowest layer should not contain any degradable matter but only inorganic and lasting substances such as gravel or stones. Garden fabric (landscaping fabric) is then placed on this layer, which prevents the soil of the upper layer from being washed away with irrigation water or rainwater. This procedure is also recommended for very tall raised beds (from 80 cm) intended for vegetable cultivation. The instructions for the subsequent layers are the same as described above.
Tip: Separate the individual layers with fleece (any type of gardening-friendly fabric) to prevent the soil from being flushed through the coarser lower layers during irrigation or rainfall.
How to fill a raised garden bed?
A raised garden bed should be filled in autumn at best. A possible alternative is early spring. Filling in autumn leaves enough time until the bed is actually planted for the filling materials to decompose and settle. If the raised bed is planted immediately after filling, it sets during the vegetation period of the plants, which can harm their growth. Although some plants, such as potatoes or kohlrabi, can manage this without a problem (additional soil can be simply added on top of them after the bed settles), other plants do not appreciate this. Moreover, filling in autumn has another advantage: the raised bed can be used as a makeshift compost pile. Gardening waste collected during the winter months can be used for the lower layers in the raised garden bed. Additionally, biodegradable kitchen waste (without animal products) and moderate amounts of pet litter can also be placed in the raised bed. The bed can accommodate an astonishing amount of organic matter, which is decomposed as the time passes. In this way, valuable and nutrient-rich material accumulates inside the bed, which is then repurposed as a source of energy for the plants planted in the bed during spring. Just one thing should be kept in mind, however, make sure that no overly salty food enters the bed, as most plants do not tolerate salty soil well.
- Autumn is the ideal time to fill the raised garden bed
- Organic matter is decomposed during winter
- The bed settles long before it is planted in spring
- Throughout winter, the raised bed can be used as a compost pile for garden waste
Importantly, even before filling the bed, lining should be thought through. If the bed is made of wood, pond or burl foil is a great choice as it can extend the durability of the wood. Likewise, with some types of bed borders, such as raised beds made of Eur-pallets, the foil can prevent unnecessary water and extra soil from getting into the bed. It is essential, to make sure that the foil does not contain any plasticisers. The foil can, for example, be applied by using a tacker on the sides of the bed. In the case of burl foil, the burl side of the foil should be placed on the wood so that ventilation is ensured. Of course, the floor of the raised bed remains free of foil because excess water must be able to drain away from the bed.
- Raised beds made of wood? Line them with pond lining or burl foil
- Burl foil allows wood to breathe even in on the inner side of the bed (through air spaces between the bubbles)
- Do not line the ‘floor’ of the raised bed so that the excess water can exit
The filling process itself is not a big deal. Layer by layer, first coarse and then finer material is added to the bed. Even if the raised bed is not completely ready, the filling can begin in the lower parts. This actually makes the process easier and less strenuous because the filling material does not have to be lifted over the bed walls. In any case, make sure that the lower layers are not too small. Otherwise, more expensive potting soil and compost will be needed in the end than is actually necessary. To prevent the lower layers from thinning out too much, the filling materials should be moist. Not completely drenched, but well moistened. This way, it is easier to estimate the correct height of the layers. During autumn, most gardens are damp enough anyway and if not, each filling layer can be dampened slightly with a watering can.
- It can be practical to start filling the raised bed before it is completely finished
- The lower layers should be laid more generously and thicker than the upper layers – this also makes it easier on the wallet
- It is recommended to use damp materials, especially for the lower layers
If the bed is filled in autumn or winter, check in spring whether the filling has settled and refill the plant substrate if necessary. If the filling takes place shortly before planting, you should in any case wait for some rainfall or water the bed vigorously several times so that it sets. The soil and compost can be added again if needed before planting or sowing.
Expert tips to avoid mistakes when filling a raised garden bed
The amount of the organic matter in the raised bed will shrink over time with certainty. The moist and warm environment of the raised bed leads to rapid decomposition and very fertile soil. However, decomposition is associated with a loss in quantity of the organic material, which is why the bed surface settles naturally over time. Particularly if the bed is filled in spring, the following measures should be considered in order to avoid a poor harvest caused by the decrease of the organic matter in the raised bed:
1. Refill soil in spring
When growing plants that can tolerate the setting of the soil in the bed such as potatoes, tomatoes or kohlrabi, their soil can be easily refilled after it settles.
2. Grow precocious vegetables and replenish the bed after harvesting
If you decide to cultivate an early spring vegetable, the bed can be refilled with plant substrate after harvesting. Spinach, salads or radishes ripen quickly and are harvested as a whole. After that, the bed is refilled and then planted with pumpkins, celery or zucchini, for example, which then remain in the bed until autumn.
3. Select non-degradable or poorly decomposable materials
As a last alternative, the bed can be filled with materials that allow sufficient drainage, i.e. a lot of inorganic, non-decomposable material, just like in the case of an herb raised bed. If the lower two layers consist of stones, coarse gravel, chippings, sand and expanded clay, these and consequently the entire bed will settle less drastically. However, bear in mind that this material does not provide fertile and nutrient-rich soil, as the conventional layering of the filling material described above does. Especially plants that consume a lot of nutrients such as courgettes, pumpkins or cabbages quickly reach their limits with their nutrient requirements.
If the soil in the garden bed sets during the vegetation period of the plants, continuous refilling with small amounts of soil has proven to be more effective than adding a large amount of soil or compost to suffocate the plant.