Maidenhair Ferns are among the most beautiful plants to grow indoors. However, few stores sell them because most people struggle to keep them alive. The basic idea is to give the ferns the right amount of soil, fertilizer, water, and light to thrive. But everyone knows that. The difficulty is in the details of the implementation. Here is some advice from a biologist.
Soil
Soil that is too dense is bad for the fern, but so is soil that is too loose.
Soil that is too dense reduces aeration of the roots and promotes rotting. When potting a plant, press down a bit on the soil but don't pack too tighly. Using planter pots with a bottom water reservoir tray will allow assessment of the packing density. If it takes tens of minutes after watering the top of the soil for water to emerge into the bottom reservoir tray, the soil is too packed. Potting soil typically typically includes some perlite, which increases aeration. Additional perlite can be mixed into the soil as well.
Soil that is too loose promotes cracks that result in water flowing down to the bottom water reservoir very quickly with an audible dripping sound. This makes it seem like the soil is adequately wet, when in fact there is just a crack in the soil. This problem can sometimes be assessed and mitigated by watering on two subsequent days, allowing the initial watering to rehydrate the soil and re-seal the cracks. If there is a major need to water on the second day the cracking may need additional approaches such as adding soil to fill the cracks or pressing down on the soil at the sides of the pot to pack the soil better. A better approach, however, is prevention: when initially potting the fern, press down a bit on the soil to make it less likely for cracks to form later, particularly at the edges of the pot.
Fertilizer
Too little fertilizer can kill a plant, but so can too much fertilizer.
Too little fertilizer deprives a plant of needed nutrients such such as phosphates and nitrogen-containing compounds. Various brands of fertilizer should be fine, such as Miracle-Gro All Purpose Plant Food, which comes as a powder. Enough powder to create a saturated solution can be added to water and then dispensed using a medicine dropper. Adding a bit of food coloring makes the fertilizer solution more visible. The best approach is to add a few drops of fertilizer to each gallon of the water used for watering, avoiding any sudden osmotic shock from adding a lot of fertilizer at once.
Too much fertilizer can kill a plant by raising the osmolarity of the soil. After a year of adding water with fertilizer and minerals in the water, some of the chemicals remains unused and build up, raising the osmotic strength of the water in the soil. Plants absorb water by osmosis, which doesn't work if the water in the soil has higher osmotic strength than the plant. To prevent this problem, flush out the plant with lots of water once a year, letting several gallons of water emerge from the fern, overflowing the bottom reservoir tray. This reduces the osmolarity to almost zero.
Water
Too little water is bad for the fern, but so is too much water. Planter pots with a bottom water reservoir tray at the bottom are good for assessing the amount of water. These planter pots have small projections of the feet of the pot into the water tray to allow water to rise from the tray into the soil by capillary action. The main watering should be once a week. When ready to water the fern, shine a flashlight into the water reservoir tray. If there is any water in the reservoir tray, don't add any more. If the tray is dry, add a significant amount of water to the top of the soil, but not so much that the reservoir tray can overflow. For these planter pots a safe amount is 2 cups of water (500 ml). While typically the fern will need to be watered only once a week, water needs are greater if the fern gets a lot of sun or the temperature is hot. As discussed above, check for water in the water reservoir tray on the day after the weekly watering to see if the tray is again dry.
Too little water results in the fern getting brown. Causes:
Too much water results in the fern no longer sending up new fronds. This may be due to insufficient oxygen getting to the roots. To avoid this, let the water reservoir tray dry out for a few days a week to keep the soil from being too water logged. Using a planter pot with aerating perforations in its bottom promotes good aeration. Avoiding packing the main part of the soil too tightly when potting the fern also helps prevent the soil from being too waterlogged.
Light
Too little light is bad for the fern, but so it too much light. Maidenhair ferns do best with only a few hours of sunlight a day.
Too much light results in the fern needing very frequent watering, and the leaf color will get more pale and yellow.
Too little light results in little growth.
A fern grows towards the light, and shows its best appearance to the direction of the light. To grow a fern with the most pleasing appearance, put it in the interior of a room, allowing it to be viewed from the same direction as the sunlight. Rotating the fern to get growth on all sides results in fronds that grow in unattractive spirals.