University of Vermont Extension 
Department of Plant and Soil Science

Summer Article


By Dr. Leonard Perry, Extension Professor

Whether you are a long-time flower gardener, or just put in your first perennial plantings this summer, you probably have a question or two, perhaps about proper planting procedures, insect and disease control, plant division, or flowerbed maintenance. A good place to find answers is in the "Frequently Asked Questions" section of my Website.

You can view the questions and answers--or post a question--at Perry's Perennial Pages at Because I receive many inquiries, especially during the growing season, I'm not able to provide individual replies. However, I will answer the most popular questions on my site. If you submit a question, please include your geographic location and a description of your growing environment.

If you are wondering what other gardeners want to know, here are a few of the most frequently asked questions:

Q: I read articles from two different Extension Service offices, which say that most perennials do well in soil with a pH of 6.6 to 7.0. Another article states that perennial plants generally do best in soil with a pH of 5.5 to 6.5. Can you put this issue to rest? (Maine)

A: This is an example of the kind of contradiction or different results that you may find in different references. Actually, in this case regarding pH or soil acidity, both pH ranges are right. Perennials are not very fussy in general about pH, a range of 5.5 to 7.5 being acceptable for most.

Of course, some such as heaths and heathers "prefer" a lower pH. Others such as delphiniums, dianthus, baby's breath, lavender, and clematis "prefer" the upper end of this range. But the word "prefer" is key (and one usually not seen in references) as many plants grow outside their range of preferences. Milkweed (Asclepias) is one that may have problems in soil above pH 6.5, and many silver-leaved perennials prefer neutral to alkaline pH.

Rather than focus on pH, if within this range, make sure plants have good organic matter. This helps "buffer" or resist swings in nutrition and pH. Most also require sufficient watering in summer and good drainage in winter.

Q: I would like to know some good, environmentally friendly ways to clear out large grassy areas (acres) to replace with wildflower seeds.  (Massachusetts)

A: To have success with wildflowers over the long term, you'll need a good seed mix with many native perennial varieties.  You also must have a well-prepared seedbed and provide the same care as if you are seeding a lawn. This can be difficult on large areas.  Of course, some people use weed whacking and mowing to lower the grass levels, then kill the grass with a weed killer.  A more ecological approach is to lower grass levels as above, till thoroughly, then cover crop for several years in order to reduce weed populations.

If a smaller area, you may wish to cover it with black plastic for at least a  year after lowering grass levels, then remove the plastic and prepare the seedbed.  Even after taking all these measures, more competitive weed seeds from surrounding areas may blow in and become established after a couple years. Reducing these populations, if possible, also will help.

Q: I need suggestions for Zone 5 groundcovers that will provide weed and erosion control on a slope in full sun. The soil has a mostly clay content and surrounds a pond. (upstate New York)

A: You have several options, depending on your needs and budget.  You might terrace the slope, either with timbers into more formal levels, or place boulders either at random or in a more organized bed fashion.  This will control erosion, and the terraces or levels can be treated as other flat areas either with grass, flowers, or other vegetation.  Otherwise, I'd suggest sowing a conservation grass mix.  This usually will have some weedy grasses, so it is not the best choice for formal lawns.

You can plant perennials such as daylilies, many perennial geraniums, catmints, ajuga (more for shade), lady's mantle, bee balm, coneflowers, rudbeckia, and ornamental grasses into this mix.  These will eventually grow and shade out grasses and provide seasonal color.  You also may wish to add shrubs such as junipers or Russian cypress (Microbiota), which will eventually fill in, too.  Many of the shrub roses are good in mass plantings, especially the rugosa ones.  You might even plant more root-invasive plants, such as the Blue Lyme grass or Ribbon grass, as long as there is no chance they will wash downstream in a waterway to spread to other areas.

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