A couple of weeks ago a reader sent in a very timely question-given the possibility of a severe drought this year: How much fertilizer should she feed her roses? How we feed the plants and trees in our gardens now can help them survive another drought year – or can contribute to their deaths.
Drought and heat stress radically change how plants react to fertilization. Drought or insufficient irrigation causes plants and trees to slow growth, the photosynthesis process and nutrient uptake. Plants basically stop growing and stop producing bright green leaves, good-sized fruit and full flowers. The first reaction of many gardeners upon seeing yellowing leaves with green veins (chlorosis), wilting, stunting and poor fruit set is to increase fertilization rates and timing. In fact, during drought and high summer temperatures we should reduce fertilization while trying to increase the soil’s ability to retain moisture and nutrients as much as possible.
To answer the reader’s question: Roses are usually fed monthly during the bud set season (March-late September) with a cup of fertilizer higher in phosphorus, the middle number on the label. The percentages of nitrogen, phosphorus and potassium in fertilizers differ widely depending on brand and type – liquid, granular or organic composts and humus. This year, it would be better to feed roses with just a half cup of a low-number, low-nitrogen granular fertilizer or a small bucket of compost after each bloom has finished, approximately five times during the season.
During drought years, it’s very important to reduce nitrogen amounts applied to plants and trees. Here’s why: Nitrogen encourages green growth and high levels of nitrogen promote rapid green growth. Forcing rapid growth during drought conditions and periods of high temperatures such as during our long, hot dry summers actually weakens an already stressed plant making it vulnerable to disease and pest problems.
Liquid fertilizers tend to be formulated with high percentages of nitrogen (the first number on the label), at least 10 percent nitrogen or even much higher in some well-known brands. Liquid fertilizers also leach out of the soil quickly, often within two to three weeks, necessitating frequent reapplications.
Low-nitrogen (around 5 percent), granular fertilizers dissolve slowly so that nutrients remain available to root systems for up to six weeks. Composts and humus contain even lower amounts of nitrogen, around 1 to 2 percent, but they provide other essential benefits. Composts and humus contain living microorganisms and fungi that can attach to roots and aid in drawing up water. Some organic granular fertilizers and some soil amendment products also contain beneficial microorganisms and fungi. Soils heavily amended with composts or humus absorb and retain water for much longer.
The best advice for gardening during a severe drought year is to plant less, fertilize less and improve the soil as much as possible. And to mulch the soil around existing plants and trees heavily with at least 3 to 4 inches of mulch to slow evaporation and moderate soil temperatures.
Elinor Teague: firstname.lastname@example.org