When the frost is on the pumpkin - or on the car windshield - it will be too late to save the houseplants that have been outdoors all summer.

In these last frost-free golden days of September, time should be allotted specifically to prepare houseplants for moving indoors. Plants need at least a week to adjust to the change of light and humidity before the furnace is turned on and the windows are closed.

The first thing to be done is to assess the condition of the plants. Brace yourself for the possibility that you may have to discard plants that have been badly damaged by insect, wind, rain or air pollution. Under usual indoor growing conditions the plants cannot be expected to be reinvigorated and restored to prime condition. Time and space are too precious to be devoted to poor specimens unless a cherished or valuable plant must be coddled from year to year.

Examine the plants and pots for evidence of insect pests. Brush or wash the outside of pots to remove soil that might harbor insects. Pick off dead flowers, leaves or other debris from the soil surface. If insects are present, you can apply a spray of a general houseplant insecticide if you have no objection to these chemicals. Work outdoors and follow label instructions closely. As an alternative, a brisk shower of lukewarm water at the kitchen sink or a spray of mild soapsuds followed by a clear water rinse is effective to remove the pests.

Plants that have grown rapidly during the summer may need repotting, especially if roots show on the soil surface or are creeping out through the drain hole of the pot. After repotting, keep the plant in its accustomed place outdoors as long as the temperature does not go below 40 degrees at night. Be sure to inspect it again for pests before bringing it indoors.

At this repotting time, rampant growers such as wandering Jew, velvet plant, Swedish ivy and pink polka dot plant will furnish trimmings from which additional plants can be grown. Cuttings can also be taken from wax begonias, German ivy, and coleus. Seedlings can be retrived from beneath mature plants of impatiens and potted for indoor bloom.

The moving in can be accomplished over a period of several days.Plants that summered in the border, under shrubbery, or on the patio can be moved to a sheltered area such as a screened porch for gradual adjustment before going indoors. Apartment dwellers should move plants indoors from balconies well before the heat is turned on so that the plants can have the benefit of good light, fresh air and natural humidity afforded by open doors and windows as long as possible.

Very soon the nights will become so cool that for your own comfort you will need to close windows and turn on the heat. Houseplants that have been outdoors for the summer months should have been inside long enough to have adjusted to the changed environment.