The proverb “If some is good, more is better” is not necessarily a useful method of fertilizing orchids. Plants will only take in the amount of mineral nutrients they need, which is determined in part by the amount of light they get, the temperatures they are grown in and the amount of water available. Just as with people, too much food can cause serious health problems in orchids — excess mineral salts can gradually become more, which they cannot tolerate. It is a gradual problem, affecting orchids that have been consistently overfertilized or have gone too long without repotting, allowing mineral salts to build up and concentrate over time.
Fertilizing
If you grow a mixed collection, one in which a variety of genera are represented, the fertilizing needs of your plants may vary a lot. The general rule of thumb — using a balanced fertilizer weekly, weakly — is a good starting point.
Many growers advise that plants be watered with plain water, then watered a second time with a fertilizer solution. This makes sure that the medium is thoroughly wet before fertilizer is used. Some mixes, particularly those containing pine bark, can be hard to wet through, and so should be prewatered with plain water and left to sit a few minutes till the medium is completely damp. This helps reduce salt accumulation and the possibility of root burn.
Most vandaceous orchids do best with copious amounts of fertilizer, and during growth may benefit from dilute applications of fertilizer as frequent as twice a week. Pleurothallids, on the other hand, do not require almost the same quantity of nutrients, and may be improved with a biweekly schedule. In addition, any orchid will need more nutrients during periods of active growth, less during periods of slower grow and possibly none during periods of not grow.
Fertilizer can be used weekly to biweekly at half or even a quarter of the strength advised by its manufacturer. When in doubt, start with a more dilute solution and watch the results on your plants carefully. Should the beginning quantity prove insufficient, the amount of fertilizer may be adjusted upward with fewer bad effects than burning the plant through over-fertilization.
There are many high-quality fertilizers available, but ensure the one you choose for your orchids is labeled specifically for their use, rather than an all-purpose garden or pot plant fertilizer. It should also include a balance of the three main elements: nitrogen (N), phosphorus (P) and potassium (K) (the percentage of each element usually appears on the label in numeric form, such as “7-7-7”; the remaining percentage is composed of inert ingredients), and also ideally micronutrients.
Water quality
Water quality is important, and contributes to the overall health of plants. Rainwater, melted snow, deionized water or reverse-osmosis (RO) water with a little of tap water added are all low in dissolved mineral salts and considered safe for watering orchids. Chlorine makes damage to orchids, but chlorinated tap water may still be used. Fill a container with the water for the plants and allow it to sit open overnight before watering. This allows the chlorine to gradually become weaker before disappearing completely . Well water and softened water are more difficult to deal with. Well water may have a high mineral content and softened water has sodium added, both of which can destroy orchid roots. They may be treated with a reverse-osmosis system to remove harmful salts.
Salt buildup
Starting with water of excellent quality is important, but salts from added fertilizers may still accumulate over a period of time. Salt accumulation appears like whitish to brownish hard outer layers on the medium and around the pot, or on the surface of the mounting substrate on mounted orchids, and can be an indication of over-fertilizing. If allowed to continue to be in the same condition, those salts will have negative effects to the health of your plant. Fertilizer salts burn and kill orchids, particularly in more sensitive genera such as paphiopedilums, phragmipediums, pleurothallids and bulbophyllums.
Fertilizer burn symptoms
Shortage of root growth may indicate an unhealthy concentration of mineral salts in the medium, on up to full fertilizer burn. If this is suspected, pour the plant and check its roots. Dead root tips, brown roots or salt crust on the potting medium surface are indications of problem. In later stages, brown leaf tips may appear, indicating burned roots. Leaves, and eventually pseudobulbs on sympodial orchids, can become yellow. If allowed to continue, fertilizer burn will finally kill the plant.
Prevention
When beginning a fertilization plan, it is safer to err on the side of a weaker solution for several months till it is determined how much fertilizer the plant needs and can use. Starting out slowly reduces the risk of destroying to easily damaged young roots, particularly on seedlings or more sensitive plants, with too strong a concentration of fertilizer.
Larger applications of fertilizer can increase plant growth in heavy feeders orchid like vandas, but when this is done plenty of water should be always available to the plant. The concentration of fertilizer salts increases when the amount of water decreases, which can make damage to the roots if they are allowed to dry too much.
Plants that need a dry winter rest period should not be fertilized for a few weeks before that rest period is started. Differently, roots might burn when fertilizer salts remaining in the pot and mix concentrate when water is reduced.
Dry or sunburned plants are most susceptible to fertilizer burn. Fertilizer salts are hard on dehydrated plants, because they are not able to take in fertilizers properly. Use a dilute solution (one quarter of the recommended strength), water well between feedings, and check regularly for salt accumulation on your pots and potting medium when your plant recovers.
Flushing media and root zones with plain, fresh water at least once a month will dissolve and remove mineral salts that can otherwise gradually increase in the pot and media and harm plants. Mounted plants also need regular leeching, and should have their mounts and roots flushed regularly. Some growers incorporate this into their care plan, fertilizing orchids at every other watering so the media is automatically flushed clean in between.
Treatment
If the damage is done and an orchid injured through overfertilizing, recovery begins with pouring the orchid, getting rid of all of the old medium and thoroughly flushing the roots with plain water to remove buildup salts. Next, do repot the orchid with fresh clean medium and a new pot. Shade the plant and shield it from temperature extremes as you would any recently repotted plant, watering often and bringing back a dilute fertilizer when the plant begins to establish.