Ask the Gardener: Is organic insecticide safe for bees?

Q: I am concerned about how safe the “organic” insecticide I have been using in my flower garden is. It is called Garden Safe with pyrethrins, but since it kills insects on contact, doesn’t this also include bees?

Durham

Pyrethrins is an organic insecticide in that it is made from the flowers of a type of chrysanthemum. After application, it doesn’t linger in the garden. It begins to biodegrade when exposed to light and air, making it a safer insecticide when compared to other bug-boppers with longer residual times. But while it is active, it certainly can kill bees. To minimize harming these beneficial insects, spray your plants late in the day or very early in the morning, when bee activity is at a minimum. And since bees are only interested in flowers, while many bad bugs like to munch on foliage, concentrate the insecticide on the leaves, shielding the blooms with a piece of paper, if you want. This surgical strike method will get messy quick if you use a hose-end sprayer, which washes down everything, so try a small spray bottle instead and only spritz plants that are being bothered by bugs. Sure, it will take longer, but if you want to be a bee buddy in the garden, isn’t it worth the extra effort?

Tips for growing azaleas

Q: I have a couple of questions about azaleas. Is it best to feed azaleas potash every July to help keep them healthy? I also wanted to be sure placing premium black mulch (made from forest products with colorant added) around plants would be all right. Is about 4 inches OK, and to put it up to the stem?

Raleigh

First, potash and phosphorus are often used interchangeably – but they aren’t necessarily the same thing. Potash, which contains phosphorus, is typically linked with wood ash, a common organic additive growers use in gardens, but it tends to raise the pH of the soil, making it more alkaline, and azaleas prefer acidic conditions. Phosphorus is necessary to keep azaleas happy, but it is typically applied as part of a complete commercial fertilizer such as 10-10-10 or 8-8-8. There are also specialized azalea fertilizers available that will not only maintain the soil’s acidity, but many are time-release, meaning you would generally only have to fertilize once in March. If you do add a complete fertilizer this July, do it sparingly, only about a tablespoon per plant.

As far as the “designer mulch” goes, back it off plant trunks slightly and keep in mind that black bark in full sun will absorb much more sizzling summer heat than lighter colored mulches.

Save a Southern magnolia

Q: I have a very special Southern magnolia that is 28 years old. I planted it in the woods (not a good idea!), so it has never gotten enough sun to bloom. But it has always been healthy and continues to grow. All of a sudden, it had a lot of yellow leaves on it this spring, and I am wondering if it was the copious spring rains, or perhaps the acidity of the soil? I have started adding coffee grounds to the base of the tree, in case the problem is acidity, but this may not help at all. Could you please help me save this special tree?

Chapel Hill

Southern magnolias (Magnolia grandiflora) like neutral to slightly acid soil, and I don’t think low pH is the problem since the woodlands and clay soils in our region tend to be acidic anyway. If the yellow leaves have green streaks in the vein areas, it could be a sign of chlorosis, which is a deficiency of iron. A soil test can help here, and if it confirms the iron isn’t in proper soil sync, treat the ground around the magnolia with an iron chelate compound (usually available at garden centers) for quick relief, or compost, which takes longer to add iron, but it will help.

You also mentioned the tons of rain we were “blessed” with this spring, and that could have caused the problem as well. Too much ground moisture can overwhelm the tree’s root system and shut parts of it down – causing it to give up leaves to cope. The cure? Well, since your magnolia is very mature and seems to be beyond the possibility of moving, dry weather will go a long way in helping. Wishing for blue skies does take control of the situation away from you, but if the area around the magnolia is prone to puddling, you might also look into installing a French drain or simple channel to help funnel future water away from the site.

This story was originally published June 9, 2015 at 11:10 AM with the headline "Ask the Gardener: Is organic insecticide safe for bees?."