IN THE GARDEN LOW HUMIDITY SHOCKS GREENHOUSE PLANTS.

Byline: JOSHUA SISKIN

Q: My problem is an angel's trumpet that I received as a gift. It is a large potted plant, full of buds that I believe will be flowers, but the old leaves turn yellow and even the new little leaves on the trunk turn yellow and fall off. I give it water every other day, and deep water once a week. Does it want more or less, and does it want sun all day or just part of the day?

- Myrna Wills,

Northridge

A: The problem you have with leaves dropping from your gift plant is a common one. Most gift plants are grown in greenhouses with highly regulated environments or, in Southern California, in mild coastal areas such as San Diego or Oxnard. When you suddenly expose these plants to the dry heat of the Valley, they go into shock and lose their leaves. This is often seen in indoor plants such as weeping fig (Ficus benjamina) and variegated dragon tree (Dracaena marginata). Having grown up in humid greenhouses, they defoliate when placed in the desiccating air of a Valley living room. That is one of the reasons indoor plant enthusiasts place mini-humidifiers in each room of the house.

Even though your angel trumpet (Brugmansia) is an outdoor plant, it probably has a pampered past. It was no doubt grown in an area where the moisture in the air was significantly greater than it is in the Valley. The best way of addressing its defoliation, like that of any other plant, is to carefully manage its water intake.

Where the humidity is high, plants do not lose water as quickly as where the humidity is low. Water exits plants by diffusion through their stomates or leaf pores into the atmosphere. The relative humidity inside a leaf is around 100 percent. Thus, the higher the relative humidity in the air outside the leaf, the less water will be sucked out of the leaf. In a greenhouse or coastal area, where the air is moist, loss of water through leaves will be kept to a minimum. But take that same plant and put it in our dry Valley and it will suddenly experience rapid loss of water through its leaves, and at a rate faster than it can replenish it by pulling up water from the soil.

A leaf - or more precisely, each leaf cell - is like a water balloon. When a leaf is healthy, its cells are fully expanded from the water inside. Remove the water from plant cells and they collapse, resulting in wilt and defoliation.

A common, and fatal, mistake made when defoliation occurs is to increase watering. The problem is that loss of leaves means that the plant's water requirement has been reduced since there are fewer leaves to take up water. If watering is increased - or even kept at the same level - standing water and rotting roots will result.

A defoliating plant should only be watered when the soil is nearly dry. Put your stressed plant in a shady spot, water it not according to how it looks but according to the dryness of the soil, and watch it slowly put on new growth and return to health.

CLARIFICATION: In my May 29 column, I wrote that Valley resident Leslie Rink removes unwanted opossums and squirrels from gardens at no charge. Leslie does not relocate animals; she raises only orphaned baby opossums and squirrels. Once they're strong, she releases them into the wild where they were first found. For information on orphaned and injured wildlife that needs rescue, call the California Wildlife Center at [phone] or visit www.californiawildlifecenter.org.