Now is the time to take "slips" or stem cuttings of plants for your indoor garden. Houseplants that are at the peak of their vigor are first-rate candidates from which to select cutting material. Garden plants which you want to keep for your indoor garden - a special coleus, begonia or impatients to be kept over for new year outdoors because of outstanding foliage or flowers - can be propagated from cuttings made now.

July and August are the months in which we can most successfully propagate houseplants. Many can be propagated by stem cuttings and the best time for this is when they are in active growth. A cutting is a piece of a branch, a growing tip, with leaves. Stems the snap off crisply will root better than flexible stems from the same plant. It is best to avoid using flowering branches.

The stems of many plants will root in water, which is a familiar method for coleus, begonia, and wandering jew, to name only a few. It is not the most dependable way to root cuttings, however, because roots formed in water are not adapted structurally to growing in soil when the plant is potted. If for some reason you prefer to start roots this way, transplant the cuttings to a growing medium when roots are no more than 1/4 to 1/2 inch long. At that stage roots can adjust more readily to growing in soil. Other plants that can be rooted in water are English ivy, Swedish ivy (Plectranthus), philodendron, pothos, aucuba, syngonium, and Chinese evergreen.

The preferred method of inducing root information on cuttings is the use of a specially prepared rooting medium. Garden centers, variety stores and nurseries handle these materials in convenient size packages.

Materials used for rooting cuttings are peat moss, perlite, vermiculite or sand. Sometimes the material is used alone, sometimes in mixture. Peat moss tends to pack: it serves better when mixed with sand or perlite in ratio of one to one. The light, white granules of perlite give good aeration for root development but do not hold water: if used alone as a rooting medium it requires close monitoring. Vermiculite alone is a good rooting medium: it holds water well and is firm enough to support the cuttings until roots form. I have used sand alone with success for miniature roses and geraniums.

Containers for propagating can be as varied as your favorite plant store or your kitchen can provide.Clay or plastic pots, small tin cans, plastic food containers and shoeboxes, disposable aluminum baking pans are all suitable. The container is only a temporary home, so a depth of 3 or 4 inches is adequate; it should have drainage holes so that it can be watered from the bottom.

Put about 2 inches rooting medium in the container. The medium must be moist, not soggy when cuttings are set in it. One method of moistening is to set the container in a bowl or tray of water until the surface of the medium feels damp to the touch, then set it aside to drain.

Make your cutting with a sharp knife or razor blade. The size of the cutting depends on the size and type of plant. A stem 3 to 5 inches long is ample. (With special care a smaller piece may be used: for instance, a very small branch snapped off near the base of a geranium can be rooted.) Cut the branch just below a node, the point where leaves grow out of the stem. Remove any leaves that would be buried when the cutting is stuck in the rooting medium.

Stick the cutting in the rooting medium to a depth of one inch and firm the medium around it with your finger.

Without roots, cuttings wilt unless high humidity is maintained. A plastic bag enclosing the container like a little greenhouse will take care of this requirement; it will also help to keep the medium moist as moisture condensed on the inside of the bag will run down and be taken up again in the medium through the drainage holes in the container. Fasten the bag in such a way that it forms a tent and does not come in contact with the cutting. A shoebox can be covered with its own lid.

Place the container where it will receive an abundance of indirect light, such as a north windowsill, and moderate warmth.

Plants vary in the speed with which they will form roots. Some of the easy ones like coleus and impatiens may root in a week. Others with rough woody stems like ivy may require a month or two. With a medium such as perlite or vermiculite, it is possible to lift the cutting to examine it: if it has not rooted, it can be replaced in the medium.

Resistance to a gentle tug is indication that roots have formed. Open the plastic bag and gradually, over a period of about a week, fold it down around the container to accustom the new plant to conditions outside the "greenhouse" environment. The shoebox lid can be tilted slightly or set at an angle.

Thereafter, the new plants can be potted in an appropriate growing mix. Set each plant at the depth at which it grew in the rooting medium. Water it and place it in a bright light but not direct sunlight for several days.

There are variations of propagating methods growers and gardeners, but the basics are these. Select a stem from a vigorous plant; use a rooting medium that will retain moisture but will also be aerated sufficiently for root development; maintain a humid atmosphere and warmth in bright light.

Follow these simple basics to increase the following plants: Phothos, Swedish ivy, English ivy, velvet plant (Gynura), polka dot plant, geranium, miniature roses, Peperomia obtusifolia, Fittonia, Euonymus, Fatshedera, Hemigraphis.

Plants that cannot be propagated by stem cuttings are palms, bromeliads, liilies, or grassy plants.

There is great satisfaction to be gained in making new plants from pieces of an old one. It is an inexpensive way to add to your own collection. You can contribute to plant sales of your garden club or bazaar. You can share your choice or cherished plant with friends or relatives or as a gift at the hospital.

When a new plant is something you have created your sense of worth and accomplishment is enhanced.