Why houseplant roots are just as important as what's happening to the leaves

Rootballs come in all shapes and sizes. Photograph: Jane Perrone.

I know they're not the traditionally 'sexy' part of the plant, but roots are the bit that really determines how well it is doing. And it's all happening largely out of sight below ground level. That's why I always love taking plants from their pots for a good old bit of root sniffing. 

A rootball should smell nice and earthy: if it smells rotten or bad in any way, there's a rot issue that you need to deal with immediately. How should roots look? The colour can vary from bright white to orange or brown, but all roots should be firm, not soft or stringy. 

While you're there, it's worth looking at the size of your rootball. If you've ever seen a cross section of a plant's growth structures above and below ground, you'll know that the roots are often as big if not bigger than the foliage above ground (here's a good one, on meadow plants). It varies of course: plants that are tree dwellers, aka epiphytes, such as bromeliads, tend to have less extensive root systems, while plants that creep around on the ground like Curio rowleyanus, the string of pearls, have a shallow root system but one that may be spread over a large area. Indeed, I believe many houseplants have been selected over the centuries for their ability to cope with a relatively small rootball (I'd love to discuss this with a botanist if anyone's out there!)

It's clearly important to think about the relationship between roots and leaves. I recently came across this Insta reel from @happyhappyhouseplant wondering why her air layered cutting of a Ficus lyrata, the fiddle leaf fig, had failed. I commented to say that the problem lay in the ratio of above ground to below ground growth: the rootball was way too small to support the amount of foliage attached to the stem, and there was no way they could pull up enough moisture to stop the leaves from wilting. This African tree species would have significant root systems in the wild, to be able to pull thousands of litres of moisture from the soil to support a large canopy. Either making a smaller cutting with fewer leaves, or removing the majority of the leaves on the existing cutting would have greatly increased this cutting's chances of success.

The simple fix when faced with the problem of cuttings that haven't quite produced enough roots is simple: raise relative humidity, i.e. how much water is being held in the air at a particular temperature. This will slow the plant's rate of transpiration - water loss from the leaves - and enable the plant's cells to stay turgid while it concentrates on root growth. The hack for this is a simple clear plastic bag placed over the cutting, pot and all, and left on for as long as needed. Try this when you pot up cuttings rooted in water for the first time, and when you are going away for a while and don't want plants to dry out. It really works! 

Let me know if you have any root insights or questions to share: as always the email is theplantledger@gmail.com.

This blogpost was taken from The Plant Ledger, my email newsletter about the houseplant scene. Subscribe here and get my free in-depth guide to fungus gnats.