reducing weight in large flower pots

Jane McGary
Fri, 11 Feb 2005 09:46:01 PST
Regarding the matter of putting various substances or objects in the bottom 
of pots to reduce the weight of the container:

The controversy about whether or not to "crock" pots, which Mary Sue noted, 
has been discussed in alpine-growing circles for decades. Although some 
still recommend it, the broad consensus at this time, as I perceive it, is 
that most plants do best when their entire root run consists of a 
homogeneous medium, so that moisture remains optimally consistent (though 
naturally the upper portion will dry out faster than the lower, and this is 
desirable for almost any plant that hasn't evolved in a really wet 
habitat). When moisture-sensitive plants are grown in very small pots, such 
as 2-inch/5-cm square, nursery growers (e.g., Rick Lupp) often fill the 
bottom quarter or even more with fine pumice, which does hold water and air.

It is important, however, that the plant does not extend its roots into an 
air pocket, and I see no reason why bulbous plants should be any different 
from alpines in this regard. There is a big difference between oxygen 
capacity in the soil and actual "pockets." A root entering the latter kind 
of space will suffer and stop growing. Result: poor performance and 
probably eventual loss of the plant. SInce the addition of styrofoam 
chunks, soda bottles, and other inert objects would create just such air 
pockets, I definitely would not recommend doing it. Judy's suggestion of an 
upside-down empty pot in the bottom would create a huge air pocket for any 
roots that came through the drain holes.

Maintaining the right level of moisture for potted bulbs is much easier 
when the pots are plunged or, at least, set slightly into some 
moisture-pervious medium such as sand. If you use clay pots, there is some 
passage of moisture through the pot itself; in solid plastic pots, very 
little, and in plastic mesh pots, a great deal. However, moisture can't be 
taken up through the drain holes unless the growing compost inside the pot 
is in contact with the drain holes; if there is a lot of inert crocking at 
the bottom of the pot, it may as well be up on a table as far as capillary 
moisture is concerned (though you still get the benefit of consistent 
temperature). Plunging pots is accepted by most skilled growers of alpines, 
and I can attest that it works for bulbs, too.

It is true that some bulbs need deep pots, and such a pot filled with soil 
mix is likely to be heavy. They are: and I heave hundreds of them, weighing 
up to 20 pounds/8 kg, out of my bulb frames every summer, carry them to the 
potting bench, dump and refill them, and carry them back to their plunge 
beds. Not to mention mixing all the soil, wheelbarrowing the used soil to 
the garden, and topping off the coarse sand in which the pots are plunged. 
It's worth it, especially this time of year, when the only heavy thing I 
have to lift is the fertilizer tank.

Admittedly, I don't grow the tropical amaryllids favored by many PBS 
members, which tend to have huge bulbs, but I do grow many Calochortus, 
which pull themselves so deep into their pots that they would soon have no 
root run at all in a crocked pot. Tulips will do this, too (with droppers), 
and many other bulbs like to go deep. Plastic mesh pots through which the 
roots can run into the plunge are a good solution, provided you fertilize 
with a soluble product so that there are nutrients in the plunge (also good 
for the inevitable self-sown bulbs that pop up there).

For very detailed discussion of this and related matters, particularly soil 
mixtures, see chapters in the new NARGS volume "Rock Garden Design and 
Construction," which also has my chapter on frame cultivation.

Jane McGary
Northwestern Oregon, USA

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