Why some plants like to be root-bound

Re-potting is a perennial gardening chore. As plants grow, their roots extend to suck up more moisture and nutrients from the soil. However, some plants can thrive even when their roots reach the limit of their pot. They only need re-potting every couple of years as opposed to every growth season. Here's why.

What does it mean for a plant to be root-bound? 

A root bound palm tree where there is more root than soil visible B137 https://commons.wikimedia.org/wiki/File:Root_bound_palm_tree_2.jpg

A root bound plant is a plant with roots that have outgrown its pot.

To check whether your plant is root-bound, remove the plant from the pot. You can do this by holding the plant gently at the root of the stems, tipping the pot upside down, and squeezing the pot gently around the middle. If there is more root visible than soil, then the plant is root-bound. 

Another sign of being root-bound are if roots are growing out of the drainage holes. The roots are extending in order to seek out the nutrients, oxygen, and water that it has exhausted in the soil.

Packed roots mean less available moisture

There are different reasons why different species tolerate having roots packed into a tight space.

One is that some plants simply don't need so much moisture to survive.

The primary function of roots is to take up moisture from the yet. When roots grow to the limits of their pot, the root to soil ratio decreases. With less free soil relative to the size of the roots, there is  less room for the soil to hold oxygen and water - two essential inputs for the plant. Usually, once the root system is visible around the edge of the potting medium, it's time to find a bigger pot.

But some plants just don't need that much water. These include species native to desert conditions where water is scarce (cacti and succulents) or the tropical undergrowth where competition for moisture, space, and other resources is fierce (dracaena, ficus, anthurium). Other drought tolerant plants include the coffee plant and spider plant.

Cacti don't need much moisture and can thrive in small pots. Their shallow root systems are adaptations to dry desert soils. Frank Vincentz https://commons.wikimedia.org/wiki/File:Teguise_Guatiza_-_Jardin_-_Hylocereus_undatus_04_ies.jpg

Orchids also appreciate being slightly root bound and very infrequent potting (only once every two to three years). This is because they have aerial roots that allow them to absorb moisture from the air. Read more about the special properties of orchid roots here. 

Packed roots mean nutrient stress

However, there are exceptions to the rule that drought-loving species thrive while being root-bound. 

Amaryllis (or hippeastrum), for example, prefer to be root bound but has relatively high water needs when producing new growth. It only needs repotting once the bulb reaches the edge of the container.

A root bound Amaryllis will reward you with blooms. This variety is called 'Dancing Queen'.  ProfDEH https://commons.wikimedia.org/wiki/File:Amaryllis-DancingQueen.jpg

Another example of a plant that tolerates being root-bound but needs frequent watering is the Christmas Cactus (Schlumbergera). This is a tropical cactus native to rainforests, not deserts, so it does not like bone-dry soil. During the growing season, it needs watering once the top inch of soil is dry.

This flowering white Christmas cactus is probably root bound. Agnes Monkelbaan https://commons.wikimedia.org/wiki/File:Schlumbergera_(actm)_11.jpg

These species will only produce flowers under nutrient stress.  Species like Clivias, Stephanotis (Madagascar jasmine), Peace Lily, or cyclamens are similar. The spider plant also tends to produce little plantlets, clones of the mother plant, only when root-bound. 

On top of oxygen and water, soil provides plants with nutrients. Usually, plants need repotting because their roots outgrow the nutrient levels available in a given volume of soil. The tendency for some plants to flower or clone themselves under nutrient stress appears to be a survival mechanism:if an individual plant is struggling to survive, then it is time to reproduce so that the species as a whole does not die off. Putting out offspring when environmental conditions are not ideal seems a counter-productive strategy. However, it may be a calculated risk: the delay between flowering or fertilization and the growth of offspring offers a chance that stressful conditions will have eased up by the time the new plants develop.

That said, these flowering plants will survive if you re-pot them. They will put glorious green foliage - they just won't flower.

Never repot any plant into too large a pot. If the soil volume is too much larger than the size of the root system, the soil will retain excess moisture and cause root rot. You should only ever repot into containers that are 1-2 inches larger than its current one.