Follow the rules if you grow from seeds

In 1980, seed houses reported record sales to home gardeners. This indicates that more and more amateurs are starting their flower and vegetable plants themselves rather than buying the transplants from garden centers.

Why? A mailbag sampling from an unofficial survey showed that:

* Half the fun is in ordering seeds and starting your own plants.

* It's more economical.

* You're sure of the variety you're getting, especially when it comes to vegetable plants.

Even so, the survey also indicated some complaints, all of which we've heard many times before.

''The seeds didn't germinate well,'' one gardener complained. ''Plants got tall and spindly before we set them outdoors,'' another moaned. ''The seeds sprouted beautifully, but then they flopped right over and died,'' still another declared.

The problems stem from some common mistakes made by first-time seed starters (and even many-time seed starters).

The following tips should help to get them off on the right track: Starting soils

Smart gardeners do what commercial growers do. They use soilless mixes containing sphagnum peat moss, perlite, and vermiculite. These so-called ''peat-lite'' mixes are sold under a variety of trade names. Those most commonly offered to home gardeners are Pro-Mix, Jiffy-Mix, and Redi-Earth.

These materials are relatively sterile; consequently they do not have any damping-off organisms that may cause seedlings to ''damp off'' and flop over at soil level. Also, they are loose, allowing the tiny roots to penetrate quickly.

They are well-drained, so water doesn't stand around the roots and suffocate the young plants. Sowing seeds

Some gardeners like to make neat rows, while others prefer to broadcast their seeds over the medium. We're of the latter school. We like to pour the envelope of seeds into the palm of one hand and, with a pencil or dibble in the other, push the seeds off as we move our outstretched palm over the medium.

Tiny seeds shouldn't be covered with the medium, but just sprinkled on top of it.

Petunia seeds are so small it takes 275,000 of them to make an ounce. Begonias are even smaller; in fact, they are dustlike.

We handle such seeds by using a clean, dry salt shaker with about half an inch of white fine sand in the bottom (don't use sea sand). Empty the packet of seed over the sand and then shake well. Sprinkle evenly over the medium. You can see where you've sown because of the sand.

Larger seeds, such as melons, can be planted one by one.

Medium-size seeds -- tomatoes, for instance -- can be done in the same manner or by the ''outstretched palm'' method.

A good way to pick up individual seeds is to moisten the tip of a pencil, put it on the medium, and then give the pencil a quick twist. Using a sieve, we cover medium and large seeds by sifting the medium evenly over the seeds. But some seeds need light to germinate; hence, they don't want to be covered by the medium.

Coincidentally, the tiny seeds are in this group. Those that like light include ageratum, balsam, begonia, coleus, browallia, strawflower, impatiens, gloriosa daisy, petunia, and snapdragon.

There are some seeds that need darkness to germinate, so these should be covered with a sifting of the medium, followed by laying a newspaper over the seed box to shut out the light. These include bachelor's button, calendula, gazania, statice, portulaca, parsley, pansy, phlox, and verbena.

Other seeds are not fussy whether they have light or not to induce germination. All they need is warmth, moisture, and a good seedbed. Germinating temperatures

Although a few seeds respond better to a temperature range of 60 to 65 degrees F., such as verbena, parsley, pansy, and phlox, most seeds the home gardener wants to germinate require a temperature range of 70 to 75 -- day and night. And there's the hitch.

Seeds may get 70 degrees during the day, but 5 to 10 degrees less at night when the thermostat is turned down in early spring. The remedy: Get a heating cable.

A heating cable is relatively inexpensive, and all that we've seen have a built-in thermostat to keep a constant 70 degrees.

You can buy a whole seed-starter kit, but you can easily make your own. Make a tray of wood with the sides 3 or 4 inches high. Place a heating cable in the bottom, being sure to loop the strands of the cable back and forth so the loops are about 3 inches apart. Remember, they must not touch.

Set a pan over the cable to act as a subirrigator (a large cake pan, cookie pan, or cafeteria tray works fine). Now you're ready to put your plant boxes in the pan.

You can make boxes of wood or buy papier-mache or plastic ones very reasonably. Watering

We like to subirrigate our seed boxes (called plant paks, by the trade), because then we can be sure the seeds do not dry out and they won't get splashed around from top watering. We keep water in the tray at all times during the germinating process.

When replenishing the water, be sure it is room temperature (about 70 degrees), or it will cool the soil and slow down the germination. We put a piece of translucent plastic (either rigid or polyethylene) over the top to keep the humidity and heat inside the box.

Don't put it in the sun as the trapped heat may ''cook'' the seeds. Seedling care

As soon as the seedlings have sprouted well, move them to a light, airy spot. If the air is stagnant, you may have to start a small fan in the room. Winter and early-spring days are likely to be cloudy, so you may need to set or hang a fluorescent light over the plants to keep them from getting spindly.

The combination of stagnant air and lack of light could also promote damping-off.

The temperature should be in the low 60s for good, sturdy growth. Transplant the seedlings as soon as they have one or two sets of true leaves. Always water immediately after transplanting, and do not place in direct sunlight for a day or two.

One of the big problems of hobbyists is knowing when to start vegetable or flower seeds indoors so they'll be just the right size when the outdoor planting date comes along.

In many areas, Memorial Day is the traditional outdoor planting date. Items such as tomatoes need six weeks from seed to setting out; thus, they must be planted by mid-April. Peppers, on the other hand, are slower growers and need eight weeks. Plant pepper seed about April 1.

The chart will help you calculate the seed-sowing dates for some of the most commonly grown flowers and vegetables. Seed catalogs are storehouses of helpful information, so write to the nurseries that put them out.

Starting plants from seed is not hard if you follow the directions.