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.
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| 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.
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| 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).