Mycorrhiza in the garden

I was probably overly harsh on the subject of Japanese maples and mycorrhiza on Friday. In practice, some benefit can be obtained from mycorrhizal treatment but overall, the difference in growth is going to be marginal, assuming that you have a healthy plant to start with. What mycorrhiza can't do is revive or improve the health of a poorly performing maple. As far as the soil microflora is concerned, boosting bacteria numbers and variety will have a far more impressive effect on the overall well-being of your Japanese maple and this is a subject I'll go into in more detail in a future post.

The reason mycorrhiza have little effect on maples is due to how they originally evolved and the types of plant they developed a symbiotic relationship with. Although it's not immediately apparent, the fine feeding roots of a Japanese maple are covered with a mass of root hairs. If you grow a seed and the initial root hits a small air pocket in the growing medium, the root hairs are easily visible. Under normal growing conditions, if you pull up a maple you won't see anything as the root hairs will be within the soil or are easily pulled off. You can see a more obvious picture of root hairs on something such as a lily bulb, particularly the roots that grow above the bulb from the base of the stem.

Root hairs serve the same purpose as mycorrhiza and specifically arbuscular mycorrhizae, as these are the symbiotic fungi most closely associated with garden plants. The root hairs penetrate the soil and provide the plant with it's nutrients in the same way as mycorrhiza do and no symbiotic relationship is required for the plant to perform 100%.

The root hairs also serve another purpose and that is as a barrier to fungi, good or bad and inoculating the soil will mean the fungal hyphae can't reach the roots and develop within them. This barrier to mycorrhiza is only present in plant species that are relatively 'new' and have developed more sophisticated forms of nutrient uptake. Acer palmatum is a very 'young' species in terms of evolution and is still unstable. This manifests itself in it's inability to produce offspring that look identical to the parent; sow a thousand seeds and you will have the majority of them unique in leaf shape and form. They may all look very similar, either red or green, but there will be subtle differences in shade of colour, the serration on the lobes or leaf size.

Where mycorrhiza can make an appreciable difference, however, is with the more 'primitive plants and I shall, hopefully, cover that tomorrow, as this type of plant is the reason for the existence of mycorrhizae.