Giant Argentine Cactus Dying

Q. I have two Argentine giant cactus bought from a big box store last summer. I planted them in the garden soil after I amended it with sand, pumice, and vermiculite to improve drainage. Recently, I saw them with the tell-tale darkening associated with root rot. When the plants were new from the nursery, all branches were firmly set in the pot and upright. Is it usual for the limbs to feel flimsy?  Or, are the remaining limbs soon to rot and bend as well.

Giant Argentinian cactus from reader

A. Argentine giant cactus is a sprawling cactus that gets about 3 feet tall. You can read more about it by googling “Argentine giant cactus” and “ASU”. It’s possible that you might expect erect stems and get sprawling stems instead. Once they are out of their confining container and surrounded by open garden, they tend to sprawl.

Sometimes pumice is used as a soil ingredient for cacti and other succulents.

Root Rot            

It’s obvious to me that your cactus has root rot. It’s either from watering too often or poor drainage or both. In my experience adding too little sand, pumice and vermiculite can cause problems. Most of our soils benefit from additions of at least 80% sand. The vermiculite is another problem. Unlike perlite, it holds water and may make the problem worse. Bottom line, you have to add a lot of sand when you added and use perlite instead of vermiculite if you want to improve drainage.

What to do? 

If you want to improve drainage for cacti then plant them higher than the surrounding soil. These type of cacti require deep but in frequent irrigation. What I mean by that is when you water them make sure the water gets at least 18 inches deep and don’t water very often; maybe twice during the summer and only once during the winter.

Get a working soil moisture meter. 

Inexpensive soil moisture meters made in Asia are sometimes defective. Consider them to be disposable. Check it in the store to make sure it’s working before you buy it. Push the probe in the soil about 3 to 5 inches deep in several locations and water it when the soil moisture meter reads an average of two or three on the scale.

Use Your Eyes

            My observations of cacti tell me when I can water and I don’t use a meter. When their outside “skin” starts shriveling, it’s time to water. Just like any xeric plant, watering them causes them to grow. If you want cacti to grow, water them more often. If you don’t want them to grow much, then don’t water them as often. Always make sure that you’re not watering them so often that it causes root rot. Opuntia from the Sonoran desert I water every three to four weeks for fresh vegetable (nopales) and fruit (tuna).