How to choose your substrate for raspberries

With the development of tunnel cultivation technology, the cultivation of raspberries in containers has increased significantly over the last decade. Growing in substrate allows for more efficient cultivation and easy testing of new varieties. However, it also brings new challenges, for example in terms of irrigation. In this article, we help you pick the right substrate for your raspberries.

Let’s find out more about the key properties of substrate for raspberries. Growers usually plant raspberry plants in 7-10L pots. The smaller the volume of growing medium in the pot, the more important the effectiveness of the substrate and control of irrigation. Raspberry planting often starts in March or April and the harvest season lasts from June to September.

Substrate protects sensitive root systems

There’s quite a limited amount of varieties of raspberry plants available so far. In Finland, the most commonly grown variety is the Glen Ample variety. This type of raspberry is known for its high yield and high-quality, visually appealing berries. However, especially in hot Summers, the root system is susceptible to diseases. With high temperatures and low air humidity this increases.

The right substrate protects and supports these sensitive roots. For this to work, it’s crucial to pick the right type of substrate. Your substrate choice will not completely prevent root diseases from occurring, but it will contribute to the well-being of the root system. You can achieve this by creating the ideal environment for the roots to thrive.

Effect of substrate structure on irrigation

Let’s take a closer look at the properties substrate has to offer. In addition to the particles of various raw materials that a substrate is composed of, it also contains air and water. The pore size determines the water and air volume in the substrate.

Plants take up water from the substrate through suction. This is the force required from the plant to take up the water present within the pore structure of the substrate through its fine hair roots. The finer the structure of the substrate, the greater the capillary suction capacity of the substrate. Larger pores, on the other hand, do not retain water due to lower suction and thus remain more air-filled. The lower suction force in larger pores also means that the plant can take up the water that is stored in these pores more easily. The first water absorbed by the plants will be the water within the larger pores.

Substrate for raspberries should provide air

So, we can conclude that the structure of the substrate affects the air space surrounding the roots and also determines the ideal irrigation frequency. For raspberries, we recommend a substrate with a high air content.

When the substrate offers a more coarse structure, the grower is able to do frequent, short irrigations. This allows the plants to use water and nutrients efficiently. A coarse substrate also enables a grower to over-water during the growing season to help cool the plants down. Especially when radiation is high irrigation is the best way to cool the plant. Excess water will simply run through the substrate.

You can also apply over-watering to even out any differences in moisture conditions throughout the tunnel and to control the conductivity (EC) of the substrate. The recommended over-watering rate during the growing season is 20-30%. Keep in mind that this depends on the type of substrate you use and not all substrates allow for this.

You can calculate the over-watering rate if you know the total amount of irrigation water applied to the plant and the amount of water passing through the container.

Have a look at the Kekkilä Professional product portfolio for soft fruits if you would like to see the options available.

Planting your raspberries in substrates

Now we have concluded that a coarse type of substrate has large pores, and as a result, offers a high air capacity. This is something to keep in mind during planting as well. When you put too much pressure on the substrate during planting, it will compact. This reduces the air space within the substrate and increases the amount of substrate needed to fill all your containers.

Make sure the young plant is supported well and remains upright, but don’t put too much pressure on the substrate. During the growing season,n the irrigation cycles will slightly compact the substrate as well. This should not offer problems. It does mean that you shouldn’t reuse the growing media to fill new containers after the season. Each time the substrate is handled or irrigated it slightly compacts further and further, reducing the air capacity bit by bit. Next to that plant diseases and pests that might have gotten into the substrate during the cultivation cycle could pose a threat. When reusing the substrate they may remain present and infect the sensitive newly planted young plants.

Fertilising raspberry plants

Usually, substrate producers add basic starter fertiliser and lime to the substrate during production. This initial fertilisation makes the early part of the season easier for growers.

As soon as the young plants start to grow, fertilisation through irrigation should start. To avoid the leaching of nutrients, keep watering times short when using a coarse type of substrate. However, if the EC accidentally gets too high, you can easily rinse out excess salts by using longer watering times.

Nutrient retention

It’s good to realise that different types of substrates retain nutrients differently. Some components, like perlite, for example, do not retain nutrients (or moisture) at all. Other materials, like peat, have a very high nutrient retention capacity. Make sure to talk to your distributor about your needs and cultivation methods when selecting a substrate for raspberries.

Monitoring substrate moisture levels during the growing season

Make sure to keep an eye on the moisture level of your substrate throughout the growing season. Check it frequently and make sure to adjust your irrigation strategy when containers become too dry or too wet.

Remember to not only check the surface of the substrate but to include deeper levels of the substrate as well. Raspberries typically grow in rather narrow, tall containers. The wettest part of the substrate is always at the bottom of these containers. This is caused by the suction force affecting the water retention capacity of the substrate. When you use a coarse substrate, it should ensure that water that has accumulated at the bottom of the containers drains out. The pores release the water and fill up with air again.

If the containers are not primed adequately and don’t let water out easily enough, the roots might get too wet. In practice you need to make sure to use the right substrate, to pick the right containers and to make sure your tunnels are sloping. Avoid having puddles of water under the containers (drainage might be required) and carefully plan your tunnel’s location and positioning.

A short version of this article was published in Puutarha&Kauppa magazine, 11/2022.

Feel free to reach out to your distributor if you need guidance on any of these subjects.