Restoring the rosarium

This year, we replaced 33 rose bushes in the rosarium. In 28 years of the rosarium’s existence, it is the largest replanting undertaken in a single year. This bold move was made possible by my research into the best approach for introducing new roses into an existing rosarium.

Replacement roses marked with poles

I grew up with the received wisdom that a new rose bush is never planted in the same soil where one has previously grown. The understanding is that the bushes would be susceptible to ‘rose replant disease’. We used to replace the soil for every new rose bush that we introduced into the rosarium. This meant that we would laboriously dig out the soil and replace it with fresh soil from another part of the garden. We also took the secondary precautionary measure of lining the hole where the new rose was being planted with cardboard before introducing the new soil to the hole.

Replacing soil somehow always resulted in spare bags of soil after the swopping exercise. It remains a mystery to me as to why one always had more bags come out of a hole than one is able to put back.

The spare bags of soil came in handy over the years though. We used them as top dressing for the lawn. After scarifying the lawn, we would also level any hollows in the lawn with the excess soil.

I decided that I needed to obtain clarity about this labour-intensive practice of replacing soil if one needs to plant new roses. Asking Google, “Why does one have to replace the soil in an existing rose bed for new roses?”, a search result returned that may change our approach to future rose gardening.

The first result was very interesting. It explained that the replacing of the soil should not be necessary if one removes all the old roots, introduce lots of garden compost and treats the roots of the new rose with Mycorrhizal fungi. Mycorrhizal fungi?

Apparently, Mycorrhizal fungi have a symbiotic relationship with plants. What they do is that they help the plants take up water and nutrients. At the same time, they act as a buffer against certain harmful microbes and pathogens that may have been left behind by the roses bushes that were there before.

All the holes dug and ready for planting

The good news was that if we introduce mycorrhizal fungi, there would be no need to replace the old soil. I immediately started researching Mycorrhizal fungi and was amazed by my findings. A network of fungal threads forms an association with the roots of the rose bushes. Becoming, in the process, an extension to their root system.

In a symbiotic relationship the fungus collects sugars from the plant while helping the plant absorb water and nutrients more efficiently. In this way enhanced growth and overall health of the plant is facilitated.

I shared the information with Keith, who agreed that this sounded like the solution for which we had been looking.

I discovered that in recent years, dried granules of mycorrhizal fungi have been widely used as an aid which helps rose bushes overcome ‘rose replant disease’. Some leading European rose nurseries even sell branded mycorrhizal fungi.

My next problem was where to find a supplier? The local manufacturer in Grahamstown had a very informative website but my email correspondence with them remained unanswered. I tried to contact an online retailer with offices in Cape Town through their website and email, but these also remained unanswered.

Determined to find this product, I at last found a retailer located in Somerset West that had stock. When Keith had to go to Cape Town, he picked up our order of 5kg of the granules on his return route to Towerwater.

Encouraged by our new solution to our rose replacing exercise, we were emboldened in our decision of selecting which rose bushes to replace this year. We ended up marking 33 roses for replacement. Some of them were of the original planting 28 years ago. Some were no longer performing making certain colourscapes appear moth-eaten. Others we were keen to replace simply had not performed as we had hoped.

New roses at Ashton Nursery

To find 33 different rose varieties was the next challenge. Our rosarium is after all comprised of a collection of 120 different varieties of rosebushes. Our first choice, Ashton Nursery, who had supplied the roses for the original planting, has scaled down the rose selection of the nursery. Their focus is currently on fruit tree production. I bought as many of the rose varieties as they had in stock that met our criteria, particularly in terms of height, growth pattern and colour. The bulk of the rose plants, Keith collected from the Tulbagh Nursery.

New roses waiting to be planted

With all the young rose plants on site, the planting could begin. With fresh compost from our own compost bins and the mycorrhizal fungi granules, the roses were put to bed.

All the roses restored in the rosarium

With our newfound garden vocabulary and learning of the microbiome of plants and more of the microbial communities and networks in the soil, we eagerly await the results of our latest gardening endeavour.   

More adventures with roots and fungi - Misty mornings and mushrooms