Root amendments, what are they and how do they work?

Hello everyone — after a bit of a holiday break, I thought I’d spend some time talking about root amendments, something that’s often overlooked but plays a big role in how plants establish and grow.

So you’ve dug your hole, mixed in some compost and planted everything up nicely… and now what? For many people, that’s where the process stops. Chucking in some compost is a good idea, but on its own it doesn’t do everything. In New Zealand, especially when growing subtropical plants that are a bit outside their ideal climate, what’s happening in the soil will greatly influence the success of your plantings, which is why commercial growers/farms focus heavily on soil biology, using beneficial organisms to support root health and improve water and nutrient uptake.

But before we get into what they do, let’s quickly explain some of the root amendments we are talking about. Mycorrhiza are a type of beneficial fungi that naturally form partnerships with plant roots. They attach to roots and extend tiny thread-like structures called hyphae out into the soil, effectively increasing the root system’s reach. Trichoderma is another type of beneficial fungus, but instead of directly extending root reach, it acts as a natural protector for roots. It colonises the soil around roots, competes with harmful fungi, and produces compounds that reduce disease pressure. Both occur naturally in soils, but their populations are often low or slow to establish in cultivated, disturbed, or new garden soils.

Mycorrhiza occur naturally in garden soils, but they don’t always establish quickly — especially in new gardens, potting mixes, or areas where the soil has been heavily disturbed. Most food crops and subtropical plants, including papaya, bananas, mangoes and citrus, form strong endomycorrhizal (arbuscular) associations, where the fungi colonise inside the root tissue itself. Inoculating at planting time places these fungi directly in contact with emerging roots, allowing colonisation to occur much faster. Once established, mycorrhiza attach to plant roots and send hyphal networks out into the surrounding soil. These fine threads increase the effective surface area of the roots, improving uptake of water and relatively immobile nutrients like phosphorus, as well as trace minerals. Mycorrhiza also help to prevent disease by forming a physical barrier on roots, competing with pathogens for space and resources, and triggering the plant’s own defence responses (more technically known as induced systemic resistance). Together, this helps reduce the impact of common soil-borne diseases like Fusarium, Pythium, and Rhizoctonia (which is very important during our wet and cold winters). We use Seacliff Organics Mycorrhizal Fungi which makes it easy to get the fungi established early, giving plants stronger roots and better stress resilience.

Trichoderma also occurs naturally, but populations can be low or slow to recolonise after soil disturbance or repeated planting. Introducing trichoderma at planting allows it to rapidly colonise the root zone, where it provides protection against key root-rot species such as Pythium, Phytophthora, and Rhizoctonia. It competes with harmful fungi for space and nutrients, colonises root surfaces, and produces antifungal compounds that help suppress disease. Trichoderma also releases enzymes that break down organic matter, making nutrients more available to the plant. An option that we use is Egmont Plantmate Trichoderma Powder 50g, which can be applied as a soil drench or mixed into the root zone when planting.

Used together, mycorrhiza and trichoderma work particularly well. Mycorrhiza focuses on nutrient and water uptake, while trichoderma protects the root zone and keeps disease pressure down. Trichoderma also helps create a more favourable environment for mycorrhizal fungi to establish, which is why commercial systems often apply both at planting rather than choosing one or the other.

Alongside beneficial fungi, seaweed fertilisers are a fantastic way to help plants establish. They contain natural plant hormones like cytokinins and auxins, as well as micronutrients, which stimulate root growth, improve nutrient uptake, and help plants cope with transplant stress. Seaweed also encourages microbial activity in the soil, which further supports healthy roots. Using a liquid seaweed fertiliser at planting — as a drench around the base of the plant gives your plants a strong early boost. A simple, effective option is Seaweed Fertiliser NZ.

How to apply:

• Mycorrhiza: Sprinkle a small spoonful directly into the planting hole, in contact with the plant roots, so the fungi can establish immediately.
  
• Trichoderma: Use a small spoonful in the planting hole as well, in direct contact with the roots, or apply as a soil drench around the root zone.
  
• Seaweed fertiliser: Mix at a rate of 20ml per litre of water and apply as a drench around the base of the plant after planting.
  

All in all, using these root amendments together gives your newly planted subtropical plants a much better start. They’re more likely to establish quickly, grow well, and cope with wet and cold winters that encourage disease or hot, dry summers that cause plant stress — all of which helps improve their long-term success and productivity.

If you have any questions, feel free to comment below. In the meantime, I will see you next week!