It is important to make sure you are meeting the nutritional needs for plants in your care. There are a number of ways to do this, depending on many factors, including fertilizer type, plant type, soil type, water regimens, etc. Below are general guidelines for meeting normal nutritional plant needs in average soils. Soils with high clay or sand content will have different requirements.
All fertilizers are identified and rated by their NPK rates. N stands for Nitrogen, P stands for Phosphorus and K stands for Potassium. These are three major macronutrients plants require for optimal health. Nitrogen (N) promotes foliage and flower growth. Phosphorus (P) promotes root and bud development. Potassium (K) promotes overall plant health. A good balance of these is important. For example, fast leaf growth without adequate root development will lead to a plant that is structurally weak.
More fertilizer is not necessarily better. In fact, the overuse of chemical fertilizers in the landscape has lead to major pollution problems in waterways worldwide. If in doubt, apply less, not more. You can always apply more fertilizer, if needed.
A word of caution, if a plant seems stressed, either due to disease, insect infestation or some other unknown cause, do not apply any fertilizer until the root cause of the stress has been identified and treated. Feeding a sick plant can lead it its demise by artificially forcing the plant to concentrate its energy on pushing new growth instead of healing.
Lawns in the Pacific Northwest are normally fertilized once a month during the growing season. A mix of slow and fast release fertilizers can be used to provide for immediate and long term nutritional needs. Because the spring can come as early as March or as late as June, you will need to pay attention to the weather before applying your first fertilizer treatment or else risk stressing the lawn due to freeze damage. The same goes for fall applications. Typically, fertilization ends no later than September, for the same reason – you do not want your lawn putting on a lot of new growth right before the onset of winter. Instead, use a winterizing fertilizer, which promotes root growth instead of leaf growth.
Most chemical fertilizers are granular, and can be applied using a spreader, either a handheld or a walk behind. Make sure you are applying the fertilizer at the rate specified by the manufacturer or you will risk burning the lawn.
Organic fertilizers are often liquid, but can also be granular or come in mulch form. Organic fertilizers are hard to over apply, but can be more expensive than chemical formulas and typically need to be applied more frequently. Whichever way you choose to go, make sure to follow any safety instructions included with the product. And, as always, use extreme caution near waterways.
After applying the fertilizer, water your lawn, unless specified otherwise (usually only in the case of some organic fertilizers). This will speed nutritional uptake by encouraging the fertilizer solution to enter the root zone. After fertilization, follow a regular irrigation schedule to prevent drought stress and to keep your lawn hydrated, which will better allow slow release fertilizers to be accessed over time.
Shrub beds are usually fertilized every 2-3 months, for a total of 4 to 6 applications per year. Fertilizers are normally applied near the root zone, but can be broadcast over the entire bed. Nutritional needs for shrubs will differ throughout the year. Usually a high nitrogen fertilizer is applied spring and summer to promote leaf and flower production, and a low nitrogen fertilizer is applied fall and winter to promote root growth. Be sure to water the area after applying fertilizer.
Annuals include plants such as Petunias, Geraniums and Tomatoes. Because of their short life span, annuals are typically fertilized more intensely than any other type of plant. Annuals are fertilized upon planting and then, depending on the plant type, every two weeks afterwards until the first freeze. This allows for the maximum amount of growth in the shortest amount of time.
Fertilizer exceptions include products such as Osmocote, which is a slow release plant food that feeds plants a regular amount of nutrients over an extended period of time. Slow release fertilizers often include a quick release component or can be supplemented with an organic quick release fertilizer to get the new plants off to a fast start. Follow manufacturers instructions for proper application rates. And don’t forget to water in your fertilizer for best results.