How to keep 'em blooming

Knowing when to fertilize isn't as easy as it used to be. Growing awareness of the impact of adding unnecessary chemicals to the environment means fixed schedules are often being exchanged for fertilizing on a flexible, "as needed" basis.

Many homeowners may be wondering whether they should fertilize spring-blooming bulbs. The old rule of applying fertilizer every spring and fall seemed to work. Abundant daffodil, tulip and hyacinth blossoms were usually the reward for spending both time and money year after year.

But the short-term advantages of routine fertilization are being weighed against are the long-term implications. Scientists are documenting problems with nitrogen and phosphorus, two key fertilizer ingredients, seeping into local groundwater and then into regional rivers and lakes, causing unpleasant problems with algae growth. These fertilizer ingredients can ultimately flow into ocean environments several states away, causing further harm to aquatic plants and animals, including commercially harvested fish.

So a little fertilizer here and there can become a problem that is hard to solve. The good news is we can maintain beautiful gardens using new approaches that help to prevent far-reaching problems.

A good place to start is focusing on the soil. If bulbs are growing in soil rich with organic matter, the natural process of decomposition is already providing nutrients. If bulbs have what they need, there is no need to add fertilizer. Soils with low fertility can be improved over time with annual applications of organic mulch in fall.

Other soils are normally high in phosphorus and may rarely need to be supplemented. Clay-based soils in northern Cook and Lake counties are often high in this bulb-boosting nutrient. Gardeners in these areas often find bulbs easy to grow without any fertilizer at all.

So what's a homeowner to do? The best approach is keeping soils organically rich, letting natural decomposition do the work of keeping soils fertile. As long as bulbs are blooming reliably and beautifully, there's no need to add any fertilizer.

But if bulbs seem to have fewer, less showy flowers, first make sure they are still growing in the right cultural conditions. What was once an ideal spot might not be in full sun anymore. And older clumps might need to be dug and divided as their leaves begin to yellow.

If properly planted bulbs are underperforming in full sun, then you might consider applying fertilizer at the rate recommended on the manufacturer's label. In beds featuring only bulbs, use a slow-release fertilizer in early fall. To avoid fertilizing twice, fertilize bulbs growing in mixed borders in the spring with a faster-acting granular fertilizer. Bulbs rarely need fertilizing more than once a year.

A soil test is a more precise way to determine if bulbs need fertilizing. When soil temperatures reach 50 degrees this spring, send a sample to a soil testing agency, and tell them you are growing bulbs. If results show low fertility, apply slow-release fertilizer in the fall, along with more organic mulch. Soil tests help to protect the environment by helping to make sure only what soil needs is actually added.

Spring-flowering bulbs are a lovely reminder of how small changes can make a world of difference, one garden at a time.