Fall is a good time to fertilize your plants in the Southwest. This will help them put on some healthy growth and store a little excess energy for the winter ahead. This year, I am urging folks in the Lower and Middle Deserts to fertilize after Fall Equinox. More about Southwest Zones here.
Fertilizer is NOT food. Plants make all of their own food! They combine sunlight, air, and water into – basically – sugar. This is called photosynthesis. Plants can get all they need to survive from soil, water, and sunlight.
Note I said survive. To truly thrive plants need some minerals – just like humans do. Generally plants get enough minerals from the soil, but sometimes some minerals are lacking. This leads to slower growth and a tendency to get sick, just like humans! In plants, insufficient minerals also leads to fewer flowers (reduced ability to bear children), and reduced ability to make the defensive compounds to ward off pests.
Fertilizer supplies the minerals that plants might need. Think of fertilizer as “vitamin supplements” for plants.
The minerals that plants need are divided into macro-nutrients and micro-nutrients based on the amount needed by the plant. All are equally essential.
Almost all fertilizers contain the macronutrients that plants need for life: nitrogen, phosphorous, and potassium, or by chemical symbols – N, P, and K. You can purchase fertilizer in more forms than you can shake a stick at, and they come with a bewildering variety of labels, but – by government mandate – the N-P-K amounts will be listed, and in that order.
Plants are complicated living beings, and need far more than NPK. I could list them all and discuss them, but it would take pages. These additional elements are what makes gardening in general – not to mention hydroponics so challenging. If a single element is lacking (like zinc) – it can meana plant fails to thrive.
The need for trace minerals is why many people are proponents of using organic fertilizers rather than synthetic fertilizers, because the required trace elements are generally in there.
When to Fertilize
Fertilizer should be provided to plants when they are actively growing. BUT – not in the middle of a searing Southwest summer. And NOT too close to the time when it might freeze. This makes life interesting. In most of the Southwest this means fertilizer in September.
Fertilizer will stimulate new growth and such tender growth is likely to be killed by frost. Fertilize at least one month before “first frost” – which is an average date and your local cooperative extension should have the information.
Plants that are actively producing flowers, fruits, or nuts should be fertilized carefully. If a nitrogen rich fertilizer is applied, plants may drop many of the flowers or fruit they have started and switch to growing leaves. Fruiting fertilizers carry their own hidden problems, in that too much may cause the plant to drop an number of developing fruit and just concentrate on making a few really large ones. Do read and follow label directions. If in doubt, use less than they call for.
Plants in the legume or pea family are stunted by fertilizer. They work with soil bacteria to take N – nitrogen – out of the atmosphere. If you give them nitrogen-rich fertilizer they often simply stop growing.
Cacti and other succulents have slow metabolisms. Always use fertilizer at half the recommended dose on them.
Fertilizer should not be applied to newly planted plants. Fertilize at least two weeks after plants are rooted in and established. Use root growth promoters if you desire, but not general fertilizer.
Less is More in the Desert
Always read and follow label directions for any chemical compound, and this includes fertilizer application. Too much fertilizer can kill the plant you wanted to help. If you are in doubt, err on the side of caution. Apply half strength fertilizer twice. Wait two weeks in between time for plants to show you that you got the dose right or wrong.
Learn More about Southwest Gardening
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More about growing fertilizer for your yard in this book Month by Month Gardening for Arizona, Nevada, and New Mexico (Cool Springs Press). This link is to Amazon and if you buy the book there the Horticulture Therapy non-profit Tierra del Sol Institute will get a few pennies – at no extra cost to you.
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