Among the many striking features are the 100m peony border, which has its own mailing list to let people know when it is starting to flower, and the Union Flag garden, which can be seen from planes flying out of Gatwick Airport.
Overseeing the gardens is New Zealand-born head gardener Cory Furness, who has also led the major redevelopment of the herbaceous border, which is to be opened this year as the Jubilee Walk.
Q. What has been the approach to the renewal of the herbaceous border?
A. It used to be purely mixed herbaceous, with apple trees as well.
We want to keep a certain amount of herbaceous and we want to include apple trees, but it is an opportunity to do something new with such a high level of invasive work. A friend of the family - Chelsea Gold Medal-winning garden designer George Carter - did the new design, which features grass bays with seating. Being a garden that is open to the public, any extra seating is always welcome.
There was ground elder all the way through, along with bindweed and a range of other problems. We have lost a lot of plants, including some very special varieties. One example is the 'Pride of Penshurst' carnation, which we have not got here any longer.
The planting will be different, with hot colours featuring at one end and cold ones at the other. It should become a very striking feature and people will see it when they drive past the gate.
The apples are all historic varieties, with nothing newer than 1750. We are not using the commercial varieties that were here before because we are not growing them for commercial reasons. But someone has to grow the historic varieties and they are far more interesting. It is traditional to grow trees in an umbrella shape to maximise the crop, but we are doing the opposite for this area.
Q. Is it easy to make changes to such a historic garden?
A. It is our responsibility to think about any changes that are made. Even when we are putting in a path, it has to fit. The oldest part of the garden has the same layout as it did in the days of Elizabeth I, although the varieties have changed. It is grade I listed, but we wouldn't want to change anything anyway. However, it's important to the family that they leave a mark as much as anyone else.
If you have the garden constantly looking the same, then it loses the sense of being a real garden. There is an echo of what has gone here before and a lot of things are done with historical accuracy. It's about keeping in mind how things look in an historic landscape.
Q. What are the challenges of being a private garden open to the public?
A. One of the challenges is that the garden always has to look finished. We open at 10.30am, which leaves a small window for the more invasive work. We try to start things such as mowing and hedge trimming very early in the day, so there is only noise in the morning. We are trying to extend the display. There has to be something everywhere or people would not come back. This has really proved successful this year and it has been a surprise to see how long the display lasted.
We get a lot of tours and a lot of coaches, but it's never so cramped that you can't move. It feels very relaxed and informal. We don't have keep-off-the-grass signs. We want people to treat it as their own garden and we have always got to keep that sense. There's a real atmosphere - it's not static and still.
Q. Have you had problems with the weather?
A. The start of the season was really dry and we were having to water the roses. It was the driest we have had in our records, which go back more than 50 years. We had just planted the herbaceous border and we were having to irrigate. Everything came really early and was in its softest growth when the temperature dropped to -4.8 degsC. Everything got frostbitten and there was quite a lot to clean up.
It felt like a wet summer, but according to the records the rain was really low. The wettest month was June, with 101mm. The problem was that the topsoil was getting saturated but it wasn't soaking through. We don't have built-in irrigation and watering is all done with hoses and sprinklers. Every day we had to do a new bit of the garden. We don't have that many taps. Some people were surprised to see this going on when the gardens were open, but we had to do it.
Q. How big is the garden team at Penshurst?
A. There are six of us working here all together. They have all been here for a very long time and they know the garden intimately. They have seen a lot of changes and they are very in tune with the seasonality of the garden. You cannot buy that level of knowledge.
2001: National certificate in amenity horticulture (level 4), Otago Polytechnic, New Zealand (based in Dunedin Botanic Gardens)
2001-02: Worked in the Rhododendron Dell, Dunedin Botanic Gardens
2002-06: Estate gardener then assistant head gardener, Little Mynthurst Farm, Surrey
2007-08: Senior under-gardener, Penshurst Place, Kent
2008 to date: Head gardener, Penshurst Place