At the start, you will probably feel adequately equipped if you have a supply of pots, a flat or two, a length of hose for watering, a trowel, and a pair of scissors. But as your growing operations expand, you will find other items, or different types of what you already have, useful and necessary.
I like a flat trowel, the kind used by bricklayers and masons it was the first type available to me and I learned to use it easily. When potting or transplanting, I draw the flat straight edge across the surface of the potting area, through the soil there and, with a twist of the wrist, flip the trowel full of soil up into the pot. (The usual scoop-type trowel cannot be used this way, and with it the job takes me longer.) With the flat edge, I also mark and press in lines for sowing seeds in flats. I draw the straight edge of the trowel across the soil in a freshly prepared flat, moving it slightly from side to side to form a "drill" or shallow planting space. These guide lines are evenly spaced every 1 1/2 to 2 inches until the whole flat has been marked off.
These round sticks of various sizes, smoothed at one end to fit the palm, pointed at the other end, are fine for making holes in soil where seedlings are to go, in either flats or benches. My own dibble is a rigid, 5-inch-long, hollow plastic tube with rubber cap to protect the palm of my hand, the type florists use on individual flowers for arrangements; a bit unconventional, perhaps, but it was used originally because I didn't have a proper dibble, and I found it so handy I have continued to use it.
A board (the dimensions of the inside of your seed flat) to which a series of dibbles are fastened can be used to mark out the whole transplanting area in one operation. Just put it in place and press down firmly into soil or sand. When you lift it up, a series of evenly spaced holes will have been made.
Almost essential is a piece of wood 4 to 5 inches square for firming soil in freshly filled flats or pots before seeds are sown. You can make a handle for this by fastening a strip of leather on top with space left for fingers to slip underneath, or by fastening an ordinary screen-door handle on top.
This tool, single or double pronged, with wood or plastic handle, is used to aerate soil and get rid of weeds between plants in bench rows or lines of pots. You can easily make such a tool: bend an 18-inch length of fairly heavy galvanized wire in half and twist the pieces together for about 6 inches below the bend—this makes a handle. Separate the two pieces of wire and bend them at right angles to the handle for the next half-inch, again at right angles for an inch, and then bend the last 1 1/2 inches of each piece downward. Wrap the 6-inch twisted handle with masking tape for comfortable handling.
Select a type with a 12-inch or longer narrow spout, equipped with a removable rose (a perforated sprinkler cap for distributing water in a very fine spray). The rose in place at the end of the spout is ideal for watering seedlings and small transplanted plants. With the rose removed, the long spout reaches to pots and flats on high shelves without splashing plants in benches below. Don't get too large a watering can, no larger, say, than 1 1/2 to 2 gallon capacity. The large ones are light when empty, but when filled with water they are heavy and unwieldy when you must raise them above shoulder height to water shelf plants.
For general watering, 1/2- or 5/8-inch diameter garden hose long enough to reach all areas of the greenhouse is essential; a spray nozzle for the hose is very useful.
Buy a large size with a fine spray-head for misting cuttings and seedlings, orchids, and other humidity-loving plants.
A good-sized one is useful for transferring soil from bins or containers to potting bench, for filling large pots and flats, and for measuring proportions when you are preparing quantities of special soil mixtures. A metal funnel with the spout corked or the handled half of a liquid bleach bottle make inexpensive scoops.
To fill benches with garden soil or to mix a batch of potting soil, you need sieves to screen out stones and twigs. If you want only a small amount of soil to pot a few plants, a standard round kitchen strainer will do. For sifting a larger quantity of soil, make a sieve by tacking a piece of 1/2-inch-mesh galvanized or aluminum wire to a wooden frame, the size determined by ease of handling. Sieves for small amounts of soil should use 1/4-inch-mesh wire.
Buy these in bundles for use as supports for tall plants, or to hold up wires or strings when you are making temporary frame supports for benched plants.
A ball or two is useful for making criss-cross supports between bamboo or cane stakes that support carnations or snapdragons.
You can fasten plants to stakes with bits of string or raffia if you wish, but commercial plant ties make the chore much easier. These inexpensive dark-green pieces of pliable paper-covered wire come in boxes of different lengths or in more economical rolls. With these plant ties, it takes only a moment to fasten a tall plant to a stake or a vigorous vine to a wall nail, and the ties are practically invisible.
It is a good idea to mark plants for identification. Use a simple name tag to help you get to know them, or obtain large labels slanted at the top for easy reading. The surface is sufficiently large to write name, date of planting, and abbreviated cultural directions for easy reference on unfamiliar plants especially valuable if you grow a variety of plants.
Get a pair of good nickel-plated hot-drop forged steel, 7 to 7 1/2 inches long, for cutting flower stems, seed packets, string. If you are wise, you will hide these from all but yourself; otherwise they are bound to disappear to be used for purposes other than gardening.
Tuck this away, too, so you will have it when you need it for separating side shoots from main stems, cutting tubers for propagating, or making cuttings of plants.
Keep a can on hand for quick spot-control of insects when you haven't time to do a thorough spraying job. A cardboard "pop-gun" type duster similar to outdoor garden dusters also has emergency value for treatment of insects or disease.
Keep these on hand for early and immediate control of mealy bugs.
Many other greenhouse aids can easily be improvised. Coat hangers, straightened out or bent, make excellent pot or basket hangers, plant stakes, or cultivators. Large plastic boxes with covers make good propagating boxes; plastic sandwich boxes can be used as pot saucers; coffee cans or small jars provide safe storage for seeds; well-washed bottles that contained window-cleaning spray can be filled with clear water for misting plants; water-filled peanut-butter jars are fine for rooting wax begonia cuttings. Just use your imagination, adapt, and have fun at no expense!