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Which plants should you let go to seed?

Let biennials like foxglove go to seed. They won’t rebloom if you deadhead them — those seeds will produce offspring next year.

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Let biennials like foxglove go to seed. They won’t rebloom if you deadhead them — those seeds will produce offspring next year.

 What to do this week Don’t let annuals go to seed. Deadhead them or they will stop blooming. Let biennials such as foxglove, hollyhock, sweet william, and forget-me-not go to seed because they won’t rebloom if deadheaded and those seeds will produce offspring for you next year. They are called biennials because their life cycle takes two years: one to sprout and a second to flower, set seed, and die. Annuals complete this process in a single year, while perennials can live for many.

Q. We need a privacy screen to block our hot tub and patio area as well as a screened porch from our neighbor’s house, a small Cape that sits high on 10 feet of fill above our ranch-style home. The houses are about 25 feet apart. We have about 20 feet to our property line. The area has partial shade, and deer frequent the area. We are looking for an arborvitae that is not too formal-looking. Green Giant arborvitae fit the requirements, but I am concerned they may grow too tall and/or wide. Other varieties have been suggested such as Spring Grove Western, which grow 20 to 30 feet tall with a 10-to-12-foot spread. Do Green Giant arborvitae typically grow to 50 or 60 feet, or is that unusual? I hate to plant the smaller variety and have it not provide the privacy I am looking for.

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CAROL EATON, Portsmouth, N.H.

A. Arborvitae are tall narrow evergreens widely used for privacy hedges. Eastern arborvitae (Thuja occidentalis) was a popular hedge until people discovered it is a deer magnet. Deer don’t like the Western native species (thuja plicata), but it doesn’t grow well here in New England. Now, however, there are relatively new crosses between T. plicata and the Japanese species of arborvitae (T. standishii) that thrive here and are also unpalatable to deer. Green Giant is the biggest, fastest growing, and best known of these. Though it usually reaches only 40 feet in height and 15 feet in width in this region, it will eventually become too big for your narrow side yard, as you fear. Spring Grove Western arborvitae and Steeplechase are similar but smaller hybrids, reaching 20 or 30 feet in height with a 10-foot width in 20 years, which makes them better choices for you. They grow about a foot a year and have thicker foliage that is less apt to brown during winter.

Alternatively, you could consider doing a mixed or “tapestry” hedge interplanted with your choice of arborvitae plus various other tall-growing deer-resistant evergreens for a contrast of textures and leaf colors. These might include Japanese cedar (cryptomeria japonica), Colorado blue spruce (picea pungens glauca), and pieris. Deer munch on the popular blue hollies, but there are some they don’t care for, including native American (ilex opaca), Foster (ilex x. attenuata), the longstalk (ilex pedunculosa) , and two hollies from Pennsylvania’s Morris Arboretum called John T. Morris and Lydia Morris. (The deer killed all my blue hollies but leave my opaca holly untouched.) Diversity will also offer you some defense against pests such as the bagworms that sometimes infest arborvitae hedges. I personally think an informal mixed planting is always more fun than a straight hedge, and if one plant doesn’t thrive or gets crowded out, you can always remove it without ruining your symmetry. That said, if what you want is a fast-growing screen, it is hard to beat the thuja plicata hybrids.

Q. I have three Endless Summer hydrangeas that have been in place in my garden for seven years. For the past couple of years, I have noticed a marked decline in the number of blooms. (I have not pruned out the old stalks until I am sure they are not going to leaf.) I am now down to one bloom per shrub. I normally side dress with aged cow manure in the spring. I’m wondering whether I should just give up on these shrubs and replant or if there is something I can do to encourage more blooms.

MARY ATWOOD, Alton, N.H.

A. Endless Summer is New England’s most popular hydrangea because it allows us to finally grow blue mop tops north of Cape Cod. It is also the garden shrub people complain about the most. The plants are loaded with sky blue blooms when you buy them in the nursery, where they got lots of fertilizing and watering. But they don’t look so good in your garden the following year after fending for themselves. Yours may be suffering from the unusually tough winter we just had. Or they might benefit from an application of garden lime. Cow manure is excellent fertilizer, but it is acidic, so maybe you need to compensate for that; hydrangeas are lime lovers. Originally billed as a miracle plant, it turns out that Endless Summer is finicky. It prefers cool summer temperatures and plenty of water. What plant wouldn’t?

Carol Stocker has been a national award-winning garden writer for The Boston Globe for 35 years. Send your questions, along with your name and the name of your community, to stockergarden@gmail.com.
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