DIY Starter Cubes for Seedlings

Seed casings are protective shells covering a dehydrated plant embryo. When seeds are exposed to moisture, they absorb water until the embryo swells enough to burst through the seed casing and sprout.

While seeds need to be moist for sprouting to occur, sprouts need oxygen as well. As such, care must be taken to not overwater and either wind up with a fungus problem or a drowned tiny plant. Also, most seeds will germinate at a temperature of 68 to 86°F, with 75°F being ideal for many plants.

Since seeds need to be kept moist—but not soggy—when germinating, some gardeners use starter cubes. These cubes are small plugs of media that allow a seed to be planted and cared for individually.

Their moisture level is easy to monitor so water can be precisely applied. Since the sides of the cubes are left exposed, the sprout roots will air prune—or, stop growing once they grow out of the media and become exposed to open air. Due to the small size of the cubes, several can be started in a single flat (just be sure to leave a space between to allow for air pruning).

Commercial cubes are commonly made with rockwool, foam, peat moss, coco or compost. Premade cubes tend to be more expensive, but easier, sturdier and less messy than homemade cubes.

Nonetheless, soil cubes can be made at home—and this is where childhood hours spent making mud pies finally pays off. They are made by pressing the soil into a cube form and then removing it. The easiest way to remove the cube from the form without breaking it is with a piston.

A simple form and piston can be made from a pair of small plastic bottles, one with the bottom removed to create a cylinder and one to slide inside it and press out the soil plug. Square plugs can be made with a small wooden post with a nub on the end, surrounded by a matching square form. The nub on the end of the piston will create a hollow to place the seed into. They can be made in a variety of sizes, but 2 in. is a nice size for starting most seeds.

Both wooden and metal forms are also available on the market. The commercial metal press I tried made less compact blocks than my homemade wooden version, but it made them much faster and seemed to hold up as well over time. My recommendation is to make a homemade form as a “proof of concept” and to try it out before spending the money on purchasing a commercially made form.

The media that is used also makes a difference as to how well the cubes perform. Seed sprouting commercial mixes are often suitable. There are many different soil mix recipes available, and what I use is a mix made with approximately equal parts coir, peat moss, perlite and compost. Additives like slow-release powdered nutrients can be added if desired, but should be used sparingly.

Now, fill your favorite grubby cup with your favorite beverage and keep it handy—this next part gets a little messy…

Add water to the mix until it forms an oatmeal-like consistency. A little water should be able to be squeezed out of a handful of the wet mix. Take the mold and press it into the mix to fill it. With a twisting motion, try to lift up the form from the soil without spilling too much of the mix, and fill any empty spaces by hand.

Place the form down where the cubes will dry, and press on the plunger (without moving the form) to compact the mix until some of the water is expressed. Then eject the cube with the plunger. The cubes will be at their most fragile right after pressing, so they should be moved carefully and with flat implements like putty knives.

As they dry, the plugs will become a little sturdier, but will remain fragile and require caution when handling. Seeds can be placed onto the cubes, loosely covered with a pinch of mix and kept moist (but not soggy) until sprouting occurs.

DIY Hygrometers for the Garden

Humidity has an impact on how quickly the cubes dry out. While a store-bought hygrometer can be purchased pretty inexpensively to display humidity, there are a few basic ones that can be made at home. The most primitive needs just a small board, a thumbtack, a few long (8 in. or longer) human hairs and a washer or coin.

Attach one set of ends of 3 to 4 hairs to the thumbtack and then attach the washer or coin to the other ends. Push the thumbtack into the board so the hair dangles the weight along one face. In high humidity, the hair will lengthen and the weight will lower. In low humidity, the hair will shrink, and the weight will rise.

By marking where the weight is on the board, humidity can be estimated. The marks can even be calibrated either by comparing the results to a normal hygrometer, or saturating the hair to take the high reading and drying the hair fully for the low reading.

To improve on this design, a pointer can be attached to a pivot and the free end of the hair attached to the pointer. Not only does this look a bit more stylish than the simple washer, but the small changes can be amplified with a long pointer for ease of reading.

Another hygrometer that can be made at home requires two matched thermometers. Take a cotton shoelace and cut the ends off. Slide the bulb end of one of the thermometers into the center of the shoelace. Using a half full glass of water as a reservoir, put the free end of the shoelace into the water, leaving not much more than the encased thermometer out of the water.

Like with a passive wick hydroponic system, the water will climb up the shoelace to the thermometer. Since evaporation from the wet bulb will have a cooling effect, the temperature from this first thermometer should be lower than the second dry thermometer. In general: the greater the difference, the lower the humidity.

For example, at 76°F on the dry bulb, there should be almost ten degrees difference for 60% humidity. At that same temperature, a difference of four degrees would indicate a humidity of 83%. This method is more exact than the hair-based example above, but it does require consulting a wet–dry-bulb humidity chart for the room temperature to calculate.

Once the cubes are well rooted, they can be planted directly into the garden or into temporary pots. Small pots can be made with PET plastic drinking cups or the bottoms of large soda bottles. A cheap soldering iron can easily make neat drainage holes, but be careful not to burn yourself and don’t inhale any fumes generated.

After the seeds have grown into plants and the plants produce their harvest, one common problem is stored garden spices becoming brittle and turning to dust if stored in conditions with too little humidity. (Lavender is handy to keep around for sachets, but a container of overly dried lavender flowers can crumble at a touch.)

Commercial products are available to maintain proper moisture levels in containers, and often use either a silica gel or a solution of propylene glycol and water. Many home remedies include adding something moist such as a fruit slice, bit of wet bread, etc., but these release their moisture very quickly and can quickly lead to molding if not carefully tended to.

Small pieces of terra cotta (like from a broken flowerpot) soaked in water release moisture at a much slower rate, so they are not as conducive to mold development. By checking daily (at least), adjusting the number and moisture of clay pieces, and opening and closing the container, moisture content can be regulated.

To use this method, one must first procure some appropriate bits of terra cotta to use. A quick, simple and cheap way to obtain clay chips to use is to take a clean clay flowerpot and break it into small manageable pieces. These pieces will work, but are generally not terribly attractive. For a more involved and craftier solution, clay spice saver coins can be made. Making homemade clay coins involves hot coals, so should only be attempted by those able to deal with hot objects safely.

The first ingredient needed for making clay spice saver coins is natural clay. Clay can also be purchased at art or craft stores. Make sure that any clay purchased is natural clay, as there are modeling clays and polymer clays that are unsuitable.

The clay should be wet enough to shape with your hands and dry enough to hold its shape once formed. If the clay is too dry, add a small amount of water and mix. If the clay becomes too wet, let the air dry it—mixing occasionally—until it reaches a more usable consistency.

Touch the tip of your index finger to the tip of your thumb in a circle. That is about the size your coins should be. Make them half as thick as one of your fingers is wide. Using a toothpick or other instrument, words or shapes can be inscribed on the coins. Once the coins have been shaped, they must be allowed to dry completely (preferably overnight). Otherwise, the high temperatures of firing the clay will turn any encased water into steam, which will burst the clay as it escapes.

Charcoal in a backyard grill burns very hot, and a piece of pottery buried in charcoal can reach temperatures high enough to glow, which will “fire” the clay. So, lay down a bed of charcoal and place the coins on top, then cover with more charcoal. Use enough coals to cover the coins, but an excessive amount is not needed.

Light the coals, and stay close enough to supervise the fire for safety. Do not disturb the pile and allow the fire to burn itself out and cool down. Once cooled, fish the coins from the ashes and wash them. Note that there might be dark and discolored spots that will not wash off. This is normal; the colors are fused with the clay and will not rub off. The coins are now ready for use, dip in water and place in spice jars as needed.

For the craft-oriented gardener, these simple DIY project ideas can be a fun way to take a hands-on look at some of the principles involved. While it might be easier to just purchase soil cubes, hygrometers, pots, etc., understanding them well enough to be able to improvise them in an emergency can be a handy skill to have.