Building a Rock Garden - The End is in Sight
by Sue Leduc

The hard work is done. All your beautiful rocks are in position. Your perfect soil is mixed and sculpted and has settled for a while. All the last adjustments for drainage and run-off are complete. Now for my favourite part - planting.

Before you go scrambling for your trowel though, it’s time to hit the books. While we can make generalizations about ‘rock plant’ growing conditions, each charming little gem will have certain requirements that must be met or all you’re going to have is a fuller compost bin. A bit of research now will save you from a lot of disappointment later in the season.

Make a list of all the plants you have on hand. Group them together according to their soil (fertility and pH), sun and watering requirements. Be very careful to research the exact species, not just the genus. As an example, some drabas need to be baked in absolutely dry soil in full sun, while others need a half-day of shade and richer, moist soil.

Planting is best done in the evening. The new plants will appreciate the coolness and humidity on their first night out, and the gardener won’t get toasted by the sun either. Take each group of properly ‘hardened off’ plants out to the garden and put them where their sun requirements will be met. Make any final adjustments to the soil mixture, such as adding compost or limestone chips. You can create extra shade by carefully positioning a few more rocks. You should have on hand a bucket of the top-dressing that you plan to use because some plants will need a nice deep collar of drainage material under them. I prefer to top-dress each plant as I go so I won’t have to manhandle the poor little thing again.

Top-dressing your rock garden is even more important than mulching your regular garden. It stabilizes the surface of the soil to reduce erosion and compaction. It retards evaporation from the soil while allowing air to penetrate easily. It keeps the soil cool. It protects the plants from excess moisture on delicate crowns in all seasons and from having soil splashed on them by rainfall or watering. And last, but not least, it ties the whole garden together and makes it look really finished. Your choice of top-dressing material should be guided by the rocks you used to build the garden, the grit you used in your soil mixture, and the specific needs of the plant.

When ‘stone dust’ comes from the quarry, it is a mix of particles of many sizes. Top-dressing should not contain any of the really miniscule particles - the dust. The first time the ‘dust’ gets wet, it will pack together and harden like concrete. At the very least, you should discard any material that will pass through a normal kitchen sieve. For the real keeners, you may want to pass the stone dust through a series of riddles and sieves so that you have buckets of ‘sized grit’ from which to choose the most appropriate size for the job at hand.

Planting a ‘rock plant’ is not much different from planting a border perennial. Tip the plant out of its pot. Gently clean the pot soil from the roots. Prune the roots if necessary. Dig a hole slightly bigger than the root mass. Place the roots into the hole while holding the plant above the surface of the soil. Fill the hole with soil, gently spreading the roots a bit and firming the soil as you go so there are no air pockets. Add the appropriate type and depth of top-dressing. Give the plant a little water, place the label and you’re done. Make sure to space the plants so that they have room to reach their mature size without becoming too crowded. Place taller plants toward the centre of the garden or in front of a blank rock face and lower or creeping plants closer to the outside or at the top of a ‘cliff’.

When handling your plants, do not hold them by their stem. Hold them by a leaf or cup them carefully around the whole plant. The plant is more likely to survive the loss of a leaf or two than it is if the stem is crushed even slightly.

Some plants will suffer greatly just from being transplanted, while others will settle in better if their roots are pruned. Generally, plants that ‘dislike disturbance’, have a small root mass (like young seedlings) or a brittle taproot (like Aquilegia) should not be root pruned. Plants that form enormous, multi-branched root systems will generally benefit from the removal of the lower third of their roots when they are moved to the garden. If in doubt, don’t root prune. It’s better to be safe than sorry.

When all the plants are safely in the ground, finish top-dressing the remaining bare soil. If you have to add more plants in the future, simply push the top-dressing out of the way, just like mulch.

Your newly planted darlings will need to be tended carefully for their first year or two. Although ‘rock plants’ are naturally adapted to require very little water and a soil low in nutrients, in the first year they will need more food and water than when they have settled in and matured. As with the rest of the garden, less frequent but deep watering is preferred to frequent light watering. A light feeding with your favourite water-soluble plant food at half strength in late June will give the young plants some added vigour to survive the worst of the summer’s heat (follow the directions on the package). You want the plants to send their roots deep looking for water and nutrients.

Weeds are not usually a problem in a mature rock garden but a brand new one will have its share. Weeds are easy to spot against the top-dressing and should be pulled promptly.

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