Common Plant Questions

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I’ve been working in Horticulture for a while now and for the past year I’ve been doing a lot of work with cultural and pest/disease related problems. I am approached with a variety of problems and questions but some questions come up more often than others.
I thought this would be a good way to answer some of them and maybe learn a few more solutions myself. I learned a lot from school, nurseries and greenhouses I’ve worked at but the best info has always been from talking with other gardeners, growers and landscapers.
Keep in mind that I’m in Michigan and a few things I’m mentioning really only apply if you have hard winters (temps here can reach as low as -15 to -20F or -26.2 to -28.8C).
A lot of times people come to me with a problem and assume it is a pest/disease problem but more often than not it is a cultural problem.

1) Inner needles/scales turning brown on conifers.

Conifers shed needles in Autumn, some shed every year and some shed every 4-5 years. It can be worse if the plant is too dry (or wet). Winter is the driest season so an evergreen exposed to wind and sun may also get burned in the winter (inner growth or previous seasons foliage). It’s especially noticeable on conifers with softer foliage. Every year I end up trimming a lot of Chamaecyparis obtusa varieties because they don’t handle winters here very well if they are in an exposed site, especially their first couple of years in the ground. It’s usually worse on the varieties with yellow foliage too.
I usually give conifers roughly the same kind of treatment I give broadleafs before winter. 1) Make sure they’re not dry heading into winter. 2) Make sure the sensitive ones have some protection from wind/sun. 3) Mulch.

2) Hydrangeas not blooming

This is the most common question I get asked and there are several possible solutions. Where I live the first thing to look at is whether or not the variety is hardy. Most H. macrophylla and H. serrata will grow here but their flower buds are not as hardy. The only way to get reliable blooms is to get a variety that flowers on new growth (remontant types such as Endless Summer). It also helps to plant them in a protected area (near the house and protected from winter wind) and to mulch them before winter. Frost can also damage flower buds. Often times we will get a warm up in Spring that forces plants to push soft new growth and then we will get a frost that damages it. Late frosts in Spring and early frosts in Autumn can kill the flower buds. When my hydrangas don’t bloom well, it’s usually not the winter but a hard frost in May. The third reason is pruning, a lot of people cut off the flower buds without realizing it. The best time to prune is immediately after flowering. If you prune them in Spring or Fall you could be removing flower buds. If you prune them in Spring, just remove any branch tips that may have died during winter.
Also make sure that they have enough sunlight and water. They need a lot of water if they are in full sun or the weather is hot. Try to water them before they are wilted. I fertilize my hydrangeas in Spring with Holly-tone but any slow-release fertilizer works great too. Avoid giving them a fertilizer with too much Nitrogen (the first number on the bag) or you’ll end up with more leaves than flowers.

3) Hydrangea blossoms from Pink to Blue

It’s not the soil pH that results in blue flowers but the amount of aluminum the plant can absorb. In order to be able to use the aluminum in the soil it needs to have an acid pH (5.0-5.5). The best way to change the color from pink-blue is by adding Aluminum-sulfate available at any garden center. If the soil is very alkaline, it will be more difficult to change the flowers blue and keep them that way. It’s also important not to add to much aluminum-sulfate because it can kill the plant. Keep in mind that some varieties don’t turn blue and some varieties have different shades and color intensity. Adding fertilizer can also effect coloring, so if you’re trying to turn your flowers blue then avoid applying a fertilizer with too much phosphorus (middle number on fertilizer bag). Phosphorus makes it harder for the plant to absorb aluminum. Try to add aluminum sulfate a few weeks before flowering. I try to add it when the flower buds are visible but still small.

4) Black Knot

I wasn’t going to add this but I get asked about it all the time in Spring. I see it all the time on plums but it can also appear on cherries and maybe peach? It looks like a warty growth on the stems and it’s most noticeable in Spring due to the wet weather. Last year, it started showing up a lot in June here because we got a lot of rain with cool temps. Anyways, I thought I’d mention it now because winter is the best time to get rid of it. Cut off infected branches and bury them, burn them or give them to an unfriendly neighbor. I’ve never dealt with a severe problem but it can cause quite a bit of dieback. Next time I see it, I’ll add a picture.

If anyone has any additional solutions to the problems I listed, please let me know. I didn’t include every possible explanation for each problem but I tried to add the ones I run into the most.

• 8 Jan, 2012
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