Window gardens

Do you have an unobstructed south-facing window that is not visible to neighbors or from the street? It might be the ideal site for your winter garden. During the winter, the sun shines low on the horizon and lights up south-facing windows. Seeds or clones can be started in the window in August, when the sun is bright but doesn’t shine directly inside the window. During the vegetative growth stage, the dark period can be interrupted with light with no effect to the plants. After September 22, the first day of fall, the sunlight is less than 12 hours a day so they will flower if given just natural light.
Flower the plants when they are about two feet tall, or any size that is convenient. To force them to flower, make sure that they get no more than 12 hours of light each day. Plants started in August and kept growing vegetatively through mid-September will mature in November.

After the first group is harvested, a new group can be started. The intensity of light is weakest around December 22, the first day of winter. Yet even as the strength of the light is waning, a sunny window in winter provides enough light for the plants to grow vegetatively. But there is a problem with the length of light, as marijuana flowers when it receives fewer than 12 hours of light. If plants were placed in the window, they would immediately start to flower under the long dark period. So the dark period must be interrupted to keep the plants growing vegetatively.

There are several solutions to this problem: use room lights during the evening so the dark period is broken with periods of light, allow streetlights to shine in, or flash warm white fluorescents on the plants for a few minutes every hour during the dark period. This is easily regulated using an appliance timer.

Another idea is to grow plants vegetatively to flowering size under electric lights. When they are flowering size, they are placed in the window where they will flower because of the long uninterrupted dark period.

In the US and southern Canada, the window system can be used during the fall and winter through to the end of March. Then the sun’s light hits the earth more directly and no longer shines into the windows. At higher latitudes, the sun reaches inside even during the summer, so the system can be used there year-round.

Some tips

If you wish to turn on room lights in the evening once the plants enter flowering, use blackout curtains so you don’t illuminate the plants. Light pollution during the dark period messes with the flowering cycle and lowers bud quality.

Plants are not sensitive to green light, so you can light the room with green light without disturbing their dark period.

The window area should get no cooler than 50degF (10degC) at night. If it does, you might keep the plants warm using a plant heat mat placed under the containers.

A lace curtain diffuses the light so that it is more evenly distributed throughout the garden. Reflective material such as a white or silver curtain or a screen placed between the garden and the room reflects some of the sunlight back to the garden.

Streetlights can interfere with the flowering cycle if they are shining directly in the window. Opaque drapes can be used to block out light pollution from streetlights and passing traffic. Moonlight, even from a full moon, isn’t intense enough to interfere with flowering.

Readers with grow questions (or answers) should send them to Ed at: Ask Ed, PMB 147, 530 Divisadero St., San Francisco, California 94117, USA. You can also email Ed at [email protected], and send queries via his websites at All featured questions will be rewarded with a copy of Ed’s new book, Best of Ask Ed: Your Marijuana Questions Answered. Sorry, Ed cannot send personal replies to your questions.